Screen Mirroring
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. - 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.
Source: https://support.apple.com/en-us/HT204289
Use AirPlay to stream or share content from your Apple devices to your Apple TV or AirPlay 2-compatible smart TV. Stream a video. Share your photos. Or mirror exactly what's on your device's screen.
AirPlay video from your Mac
in the video playback controls, then choose Turn Off AirPlay.
Mirror or extend your Mac display
With Apple TV or an AirPlay 2-compatible smart TV, you can mirror the entire display of your Mac to your TV or use your TV as a separate display.
To change the size of your desktop mirrored on your TV, click
in the menu bar. Then select Mirror Built-in Display to match the size of your desktop or Mirror Apple TV to match the size of your TV.
AirPlay also lets you use your TV as a separate display for your Mac. Just click
in the menu bar, then select Use As Separate Display.
To stop mirroring or using your TV as a separate display, click
in the menu bar, then choose Turn AirPlay Off. Or press the Menu button on your Apple TV Remote.
Learn more about mirroring or extending the display of your Mac.
iMore: How to mirror iDevices on a TV
Use AirPlay to stream or share content from your Apple devices to your Apple TV or AirPlay 2-compatible smart TV. Stream a video. Share your photos. Or mirror exactly what's on your device's screen.
AirPlay video from your Mac
- Connect your Mac to the same Wi-Fi network as your Apple TV or AirPlay 2-compatible smart TV.
- On your Mac, open the app or website that you want to stream video from.
- In the video playback controls, click
. - Select your Apple TV or smart TV. Need help?
in the video playback controls, then choose Turn Off AirPlay.Mirror or extend your Mac display
With Apple TV or an AirPlay 2-compatible smart TV, you can mirror the entire display of your Mac to your TV or use your TV as a separate display.
- Connect your Mac to the same Wi-Fi network as your Apple TV or AirPlay 2-compatible smart TV.
- On your Mac, click
in the menu bar at the top of your screen. If you don't see 
, go to Apple menu > System Preferences > Displays, then select "Show mirroring options in the menu bar when available." - Choose your Apple TV or AirPlay 2-compatible smart TV. Need help?
- If an AirPlay passcode appears on your TV screen, enter the passcode on your Mac.
To change the size of your desktop mirrored on your TV, click
in the menu bar. Then select Mirror Built-in Display to match the size of your desktop or Mirror Apple TV to match the size of your TV.AirPlay also lets you use your TV as a separate display for your Mac. Just click
in the menu bar, then select Use As Separate Display.To stop mirroring or using your TV as a separate display, click
in the menu bar, then choose Turn AirPlay Off. Or press the Menu button on your Apple TV Remote.Learn more about mirroring or extending the display of your Mac.
iMore: How to mirror iDevices on a TV