We covered the Windows key previously. If you hold that key down and then press the “d” key, you’ll see the desktop. Press Windows+D again and your application windows will reappear.
Note: Some people like the Windows+M shortcut, which minimizes all applications. Windows+D and Windows+M are very similar, but I prefer Windows+D because it’s a toggle and because not every application can be minimized. So Windows+D is just a little bit more generally useful.