From the official site:
Arrr! - This one is often confused with arrrgh, which is of course the sound you make when you sit on a belaying pin. "Arrr!" can mean, variously, "yes," "I agree," "I'm happy," "I'm enjoying this beer," "My team is going to win it all," "I saw that television show, it sucked!" and "That was a clever remark you or I just made." And those are just a few of the myriad possibilities of Arrr!