1) a way of harmless roaring at someone online without scaring them, rather making them laugh or 2) when pronounced out loud it's used to convey the lack of intelligence you yourself has, especially when used randomly and/or for no reason at all.
person 1: nice fupa buddy, rarg!
person 2: (?) are you retarted?
aim_guy: rarg, RARG!!!
aim_girl: whoa, i was scared at first, but now realize that you're just being silly.... you're dumb.