https://support.apple.com/en-us/HT202618
It's a mode where the Apple TV display instructions on how to connect with airplay and a pin code for authentication. It's really a handy feature in combination with Direct Wi-Fi, since it allows anyone in a conference room with a Mac or iDevice to present things.
We use it in meetings (though I prefer hooking up to by cable, since Airplay doesn't do 4k yet for 'screen sharing').