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Most PCs allow for two screens (or monitors) to be connected. This is also the case for laptop PCs. The first screen is the built-in one on a laptop or the screen sitting on the desk for a desktop PC. The second screen (especially in class rooms) may be the projector.
There are two ways of using the second screen. One is to have it as an exact mirror of the main screen. This means that you effectively have one screen and it is duplicated to the projector. The other method is to extend the display so that it is a true second screen. Each of these has its own advantages.
If you mirror the display, then it is easier to know what is being displayed as it is identical to what is shown to you on the first screen, i.e. the one in front of you.
If you extend the display, then you can control what the audience (the class) sees more tightly. For example, you can run a PowerPoint or Google Slides presentation on the projector (in "presenter mode") and have your notes and other material only showing to you on the first screen.
On Windows: