To move a window means to drag it around the computer screen, using the Mouse Drag technique described below. Only non-maximized (custom sized) and maximized (full desktop screen sized) windows can be dragged (moved) around simply because a window that is in Full Computer Screen mode occupies the whole of the computer screen whereby it has no Menu Bar and therefore no move/drag or re-size options available.
In the example below the window I want moving belongs to the TextEdit application, which is currently sitting behind the Safari web browser application window. In order to move (drag) it around, and see/edit its content, I first need to get it in front of the Safari window. This is done by clicking on its Title Bar.
Fig 1.0 Click on the Title Bar of a window to bring that window to the front of all other windows
When you have brought a window to the front of all other windows the next thing to do is move (drag) that window into its new position. So click on its Title Bar again but this time keep the left mouse button (click) held down, as you then move (drag) the mouse pointer leftwards/rightwards and/or downwards/upwards. Doing this will move the mouse pointer and the window, together, around the desktop screen.
Fig 1.1 With the left mouse button held down, on the Title Bar, move (drag) the selected window around
Fig 1.2 Move the window Left/Right, and/or Up/Down until it reaches its new desktop position
Once you are happy with the new desktop position of the window release the left mouse button. In this example I moved (dragged) the mouse pointer leftwards and then downwards, with the left mouse button still held down on the window's Title Bar, until the TextEdit application window was underneath the Safari web browser application window. I then released the left mouse button.
Fig 1.3 When you are happy with the selected window's new desktop position release the left mouse button
Although the drag technique is straight forward, it does take practice to do the more skilled positioning. The drag technique is used in many areas of OS X (Sierra), such as dragging a window, dragging desktop icons and dragging picture files. Practice does make perfect!
One thing to note here is that although a window cannot be dragged when it is in Full Computer Screen mode, because it does not have a Menu Bar and therefore no Title Bar, it can be dragged off the desktop screen (out of its boundaries/edges) when it is of a maximized size or custom size. This is good because it technically gives you more desktop space for your windows - Some windows might be sitting perfectly inside the desktop screen whilst others will be half sitting outside for example with just enough content viewable for you to know what application/window/folder you are working with and have selected.