Live streaming lingo for going into the chat of a sighted/featured/mentioned streamer. It is typically frowned upon because of the people who will go into other chats to spread toxicity upon, per example, dying to another streamer in an online game.
Chatter: Fuck that guy! He's so lame, I'll go tell them off!
Mod: No crosschatting.
Person 1: "Wizzards are so cool!"
Person 2: "True, but wizard is spelt with a single z."
Person 1: "Damn. You're right, but isn't blizzard spelled with 2 z's."
Person 2: "You make a fair point."