Use AirPlay on your Mac
Show whatever’s on your iMac on the big screen using AirPlay Mirroring. To mirror the iMac screen on your TV screen or to use the HDTV as a second display, connect your HDTV to Apple TV and make sure the Apple TV is on the same Wi-Fi network as your iMac. You can also play some web videos directly on your HDTV without showing what’s on your desktop—handy when you want to play a movie but keep your work private.
Mirror your desktop using AirPlay Mirroring. Click the AirPlay icon 
in the menu bar, then choose your Apple TV. When an AirPlay display is active, the icon turns blue.
In some cases, you can use an AirPlay display even if you’re not on the same Wi-Fi network as Apple TV (called peer-to-peer AirPlay). To use peer-to-peer AirPlay, you need an Apple TV (3rd generation rev A, model A1469 or later) with tvOS 7.0 or later.
Play web videos without showing your desktop. When you find a web video with an AirPlay icon , click the icon, then select your Apple TV.
Tip: If the image doesn’t fit your HDTV screen when you mirror the screen, adjust the desktop size for the best picture. Click the AirPlay icon , then choose an option under “Match Desktop Size To.”
Apple TV is sold separately at apple.com or your local Apple Store.
Learn more. To learn more about AirPlay, see Use AirPlay to stream what’s on your Mac to an HDTV in the macOS User Guide. To learn more about using a second display with your iMac, see Use an external display with your iMac.