Screen mirroring is a setting that can be enabled when more than one display is connected to a computer at once. This will duplicate what is being shown on your main display and is most commonly used for demonstrations and presentations in conjunction with a TV or projector. 

Does Screen Mirroring Using HDMI Cable?

Setting up screen mirroring is as easy as connecting an HDMI or DisplayPort cable from the monitor or TV to your computer and then enabling screen mirroring in your system's display settings. Display settings can be accessed by navigating to Start > Settings > Display on Windows and System Settings > Display on MacOS.