Use AirPlay to stream what’s on your Mac to an HDTV
AirPlay Mirroring lets you send what’s on your Mac to an HDTV wirelessly with Apple TV. If your Mac supports AirPlay Mirroring, an AirPlay status icon 
appears in the menu bar of your Mac when an Apple TV is on the same network as your Mac.
Before you begin, see the Apple Support article Use AirPlay to stream video or mirror your device’s screen to find out if you can use AirPlay Mirroring with your Apple TV and Mac.
Make sure your Mac and Apple TV are on the same network.
Click the AirPlay status icon
in the menu bar, then choose your Apple TV. Note: If you don’t see the AirPlay status icon in the menu bar, check that your Apple TV is set up correctly.
To set whether your desktop matches the size of the Mac screen or the TV screen, click the AirPlay status icon then choose This Mac or your Apple TV. When you’re using AirPlay Mirroring, the AirPlay status icon in the menu bar is blue.