You can mirror your iPhone, iPad, iPod touch, or Mac screen on an Apple TV or an AirPlay 2-compatible smart TV.
However, if you want to cast your whole screen - with all of the windows or applications you may have open - then you’ll want to use mirroring. If you use an iPhone or Mac and you want to cast a video from a specific app on your iPhone or Mac to your TV, you can use AirPlay.