AirPlay Mirroring enables you to display your iPhone screen on a television via an Apple TV box. Make sure both devices are turned on. Then open Control Center on your iPhone and select the AirPlay option.
Choose Apple TV and turn on the Mirroring option. The iPhone screen will now appear on your television. To stop this, return to Control Center, and turn off the Mirroring option. Note that only the iPhone 4S or later has this capability built in.