The equivalent of "yes" but used with close friends. Also shows some excitement in your reply, i.e. you would use this in place of "hell yes" not "yeah".
Drew: "Do you want to go to the casino tonight?"
Paul: "YERS!"
Drew: "Would you like to get some roadies for the way there?"
Paul: "YERS!"
Drew: "Yers bro, I'll pick you up soon."