"that shit" is vulgar slang for "that" or "it". here, "shit" is similar to "[那個］東西"。 "That shit is great!" = "That is great!" “To rock it" means to do something with a lot of energy. It comes from "rock and roll" music. We say if a band is good, they "rock". So we have a common slang word in english. "to rock", which means something is really good. "That new TV show rocks!" "This computer rocks!" etc so "rock that shit" means more or less: "do that thing really well".

I'm guessing it means she's rocking her body back and forth However, it doesn't matter.......the lyrics of this song will only deteriorate your knowledge of the English language