It can also refer to an item of clothing or a particular fashion look. It can be used to describe someone who is doing really well at something too.
(for a video game it can also be another way of saying 'Im running with this...' for example for C.O.D it's like saying 'Im running with this class...' Instead you say 'Im rocking the sniper class for this game...' etc.)
-Hey man how are you doing so good?
-I'm rocking the sniper class and the chopper gunner for my killstreak thats how!
To describe someone doing well: (eg. playing rugby in real life)
-Hey i've scored 5 tries so far!
-Wow man you are seriously rocking it today!!
For clothes: (eg. Fashion, outfits etc.)
-Dude you look great for the fancy dress party tonight! That looks awesome!
-Yeah thanks man, I'm rocking the 'Elvis Presley' look tonight!