To chatter is to talk lightly or casually — to shoot the breeze or chitchat. You might chatter with your workmates about the weather or where you'll eat lunch.
You probably chatter with your friends all the time without even thinking about it, gossiping or talking about unimportant things. You can call the talk itself chatter too: "The chatter in the cafeteria was so loud I couldn't hear myself think." In the 13th century, it was chateren, from an imitative Middle English root.