Usually goes with the word 'it' afterwards, you must make clear what 'it' is. To be rocking (it) means that you are using something or doing something, or 'going with' something, be it a weapon on a video game or an object or thing in real life. It depends on what 'it' is. you must make clear what it is that you are 'rocking,' in order for 'rocking' to be used as a phrase in context and everyday conversation.
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.)
For a video game: (eg. playing cod)
-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!