If you are using multiple displays, you can change how they behave by pressing the Windows key + P, a flyout will then appear on the right hand side of your screen, see below for a definition of what each option does:
PC screen only will only show the desktop on your main display (the main display can be changed by right clicking on your desktop and then clicking on Display settings, and then click Identify and from there you can rearrange and select which display you want as your primary).
Duplicate will show the same thing across all displays
Extend will allow you to have your dektop on your mail display, and other apps and programs on the other display(s) as you can simply drag and drop what you want to display on the other screens.
Second screen only will do the same as PC screen only, but on the nominated second screen.
