To scroll through the contents of a window means to move its display area (contents within its window pane) either up, down, left or right by using one of the scroll bars or scroll buttons located on one of its borders (window frame) - That's if a specific application supports scroll bars and/or scroll buttons of course and therefore allows scroll bars and scroll buttons on its window(s). Not all applications support scroll bars and scroll buttons, especially if they are using native OS X (Sierra) windows, but that doesn't mean you cannot scroll contents using other methods. You can.
Below are the two scroll bars (also known/spelt as: Scrollbars) you will find on many, common application, windows within the OS X (Sierra) operating system. They are normally accompanied by four directional scroll buttons (left, right, up and down) but Apple took them away from the OS X (Mountain Lion) operating system for some strange, unknown, reason(s). This decision has p***ed off many in the Apple Mac community, quite understandably, as they were great for scrolling a page of text one line at a time for example. Anyway, not to worry. As said above, there are other ways to scroll the contents within a window.
In this first, and main, example I have opened a text document (.rtf file) in order to show you how the vertical scroll bar works; which out of the two scroll bars I would say is the most used simply because most web page contents, folder contents, document contents and so on utilizes the height of a window as opposed to its width.
A scroll bar will only appear on a window's border if the contents being displayed in that window takes up more than one display area (i.e. more than one page). So in the example below there is no horizontal scroll bar simply because the contents (text), width-wise, does not overlap the right edge of the white display area (window). If it did, if the text went two or more pages to the right, a horizontal scroll bar would be placed at the bottom of the window; on its bottom border (frame).
When you are just viewing the contents displayed inside a window the scroll bar(s) will be light grey in colour (Fig 1.2), but as soon as you hover the mouse pointer over a scroll bar it will turn dark grey (Fig 1.3).
Fig 1.2 This window is displaying the contents of a text document - Only the vertical scroll bar is available
Fig 1.3 The vertical scroll bar turns dark grey when you hover the mouse pointer over it
At this point (Fig 1.3 above) I have not clicked on the vertical scroll bar. The mouse pointer is merely hovering over it, ready for action. As soon as I do click on it (Fig 1.4 below) it can be moved (dragged) upwards or downwards. In the example below I have moved (dragged) the vertical scroll bar downwards by about four pixel lines. This was achieved using the same drag technique as moving a window:
I first clicked on the vertical scroll bar and kept the left mouse button (the click) held down while I then moved the mouse pointer (physical mouse) downwards, towards the bottom of the page. As I moved the mouse pointer downwards the vertical scroll bar followed the mouse pointer and therefore moved downwards too. Hence, I dragged (moved) the vertical scroll bar down the page (window border/frame).
As you scroll contents (i.e. text) through a window's white display area the window moves that contents through its white display area one pixel line at a time. Or put another way; In this case it scrolls the page downwards one pixel line at a time. A pixel is a dot. So if your computer screen is 1024 pixels (dots) wide by 800 pixels (dots) high it means the top of the page that was displaying pixel lines 1 to 1,000 for example will now be displaying pixel lines 2 to 1,001. Don't get a pixel line (i.e. 1024 dots wide by 1 dot high) confused with one line of text because a line of text could be 12 pixels (dots) high if the font used is 12px for example. In other words your text will be a certain amount of pixels (dots) high, such as 12 but definitely not 1.
Fig 1.4 Click on the scroll bar and keep the LMB down. Now move the mouse pointer (scroll bar) downwards.
It is very important not to let go of the left mouse button (click) as you drag (move/scroll) the vertical scroll bar downwards (or upwards) because doing so will stop the scrolling. You need to keep the mouse pointer straight and steady, otherwise you might become frustrated at the fact of having to re-click on the scroll bar so many times. As a beginner dragging a scroll bar takes practise - Many beginners tend to move the mouse pointer leftwards (off the scroll bar), instead of straight down, when first attempt the drag (scroll) technique whereby they lose the scrolling, because the mouse pointer is no longer on the scroll bar, and end up pulling their hair out or swearing!!
In the above example (Fig 1.4) I have scrolled the contents (text/page) about 4 Pixels (Dots) downwards by moving (dragging) the vertical scroll bar downwards very very slowly. The line at the top (Click Wise Computer Services) is half hidden (disappearing) whilst the line at the bottom (What about a re-installation?) is just becoming visible. If I continue to scroll the contents more of it becomes visible (Fig 1.5 below).
Fig 1.5 As I drag the vertical scroll bar downwards, slowly, I can see more of the contents (text).
You don't need to drag the vertical scroll bar too fast, otherwise you will find yourself dragging it upwards and downwards all the time just to view contents you have whizzed by and missed! This doesn't mean you should drag like a tortoise, it just means when viewing contents for the first time you should take things easy. Once you have viewed a chunk of the contents you can then skip past it next time around by dragging the vertical scroll bar a little faster. Practise makes perfect.
Fig 1.6 When you are better at dragging the vertical scroll bar you can drag faster if you wish
When you have dragged the vertical scroll bar to a position on the window's border (frame) you like, such as the place in your text document where you want to continue reading from, only then should you let go of the left mouse button. In the example below I have dragged the vertical scroll bar down to the bottom of the window's border (frame) and therefore to the bottom of the text document. Releasing the left mouse button also makes the vertical scroll bar turn light grey again to denote it is no longer active (in use).
Fig 1.7 Release the left mouse button when you have reached a place within the contents you want to view
Another common problem with the absolute beginner is that they tend to click inside the white/grey-ish space above the vertical scroll bar, between the top of it and the top of the window's border (frame). And the same applies to the white/grey-ish space between the bottom of the vertical scroll bar and the bottom of the window's border (frame). The problem with this is that that space has a function behind it. For example. If I click on the white/grey-ish space above the vertical scroll bar it will scroll the contents of the text document up by one page. In this example (Fig 1.7 below) to the top of the text document (first page).
Fig 1.8 Clicking inside the white/grey-ish space above the scroll bar will make the contents scroll up by one page
Fig 1.9 The contents has been scrolled up by one page, to the top page in this example.
Although this section has concentrated on the Vertical scroll bar, the Horizontal scroll bar works in exactly the same way. In these next two examples I could scroll the contents of the window (folder) vertically or horizontally simply because it has a vertical scroll bar and a horizontal scroll bar available. You don't need to use the drag technique all the time either. You can use the Scroll Wheel located on the top of your physical mouse too.
Fig 1.10 With this window you can use its horizontal scroll bar to view the Folder/File Sizes and Types
Fig 1.11 With this window you can use its vertical scroll bar to view more Folders and Files
Moving (Dragging) the vertical scroll bar using your mouse's Scroll Wheel is ideal for fast scrolling; when you want to quickly skim through a document for example. Moving (rolling) the scroll wheel upwards scrolls the contents of a window downwards and moving (rolling) it downwards scrolls the contents of a window upwards. Furthermore, if you press the SHIFT keyboard key down while you move (roll) the scroll wheel upwards the contents of the window will scroll to the right. Equally, if you press the SHIFT keyboard key down while you move (roll) the scroll wheel downwards the contents of the window will scroll to the left. Note though; You must have the window, and more precisely its contents, active (selected) when you use the scroll wheel. Otherwise the contents is not guaranteed to be scrolled.
You can also use the UP and DOWN keyboard cursor keys (up and down arrow keys) to scroll the contents of a window one text line up or down. Likewise; You can the LEFT and RIGHT keyboard cursor keys (left and right arrow keys) to scroll the contents of a window one character to the left or right. Depending on the contents and application these keyboard keys may make the contents scroll further (i.e. two words to the right). You will not really know until you start using specific applications.
The PAGE UP and PAGE DOWN keyboard keys, as their names suggest, scroll the contents of a window one page upwards or one page downwards whereas the HOME and END keyboard keys take you to the top or bottom of a page (window pane). As you may of noticed; Scrolling is really used for the purpose of scrolling text within a window and for scrolling through folder contents such as Photo files. Scrolling is taken to the extreme slightly when you begin to use an application that has multiple windows (known as: Window Panes) such as the e-mail application called Mail.
Fig 1.12 A Window split into multiple Window Panes, all with vertical scroll bars
In the above example the main window of the Mail application has been split up into three window panes with the left-most window pane listing the e-mail accounts and associated e-mail folders. The middle window pane is listing the e-mail headers (titles and short descriptions) while the right-most window pane is displaying the e-mail message belonging to the e-mail that is selected in the middle window pane. Two other applications that use multiple windows (window panes) are Microsoft Word 2011 and FileZilla (an FTP Client / Website File Uploader/Downloader application).
As you go through the lessons on this website, such as the lessons in the E-Mail and Preferences sections, you will bump into more examples of using Windows whereby you will learn about Tabs as well.