From the YT channel page:

“In this live stream we’re going to play Agar.io or Agario I don’t know how to say it. Check it out at http://Agar.io and don’t forget to like this video and subscribe if you enjoyed it!
Agario/Agar.io is a multiplayer game kind of like Osmos. The object of the game is to be a circle and eat smaller circles while bigger circles try to eat you. The cool thing about this is that it’s multi-player, which ups the challenge quite a bit! I had a lot of fun and I can guarantee that I will be playing this game more in the future.”