When connected to an external display - e.g. a monitor or a projector - your computer can be set to 'project' different things to that display.
To change these settings, hold the Windows key and press P to bring up the menu. Tap P with the Windows key still held down to cycle through the options. You can also use the mouse or the up/down arrow keys.
PC Screen Only will disable the external display, so for example with a docked laptop, this will disable the monitor and only display on your laptop's screen.
Duplicate will show the same thing on both screens, which is useful when presenting. Be aware that because the two screens will usually be different sizes, the picture may look slightly fuzzy or blurry.
Extend allows you to have different things on each display. So you could have your emails open on one screen and be working on a document in the other.
Second Screen Only will disable your main display, so for example with a docked laptop, only the monitor would be active. The laptop screen would go blank.