Yes, we use this in American English. This is hard for me to explain though but I will give it a try and if another native American English speaker disagrees, they can comment on my answer. Anyhow, it can mean to do something very well. Say you are in a competition and it is your time to perform, you could tell your teammates "OKAY, everyone! Let's rock it!" to mean "Let's go out there and perform so well that we beat the competition!!!!" It can also mean to party / dance hard though. For instance, "Let's rock it hard to Lady Gaga's new song!" = "Let's party / dance hard to Lady Gaga's new song!"

It means to do your best at something or to have a fun time in some event or activity.