colloq. phr. - expression used to convey admiration, gratitude, and/or affection. Often appears in conversations when one has impressed the other; whether it be by a show of one's talent or skill, or generosity and kindness at heart. Also effectively used in casual conversation as informal expression of love, especially when in the presence of others.
After a particularly "attentive" evening of lovemaking, the next morning Laura said to Chris, "You rock", while amongst their group of friends, though she was really saying "I love you so much and you are the MAN"