In this case, "rock" is part of an exclamation. In conversational English, to say that someone "rocks" is to say that they are cool, or that they are "awesome." The original reference is to rock music, and how "cool" it was to be in a rock band or to be a rock musician. Now, when someone says "You rock!", it's giving them a compliment. Here's another example. Say I just agreed to pick my sister up from her doctor's appointment because she doesn't have a car. She might thank me by saying: "You rock!" Hope that helps!