Freddie: I’m going to organize the biggest party of all time.
Mick: Dude, you rock! When is this party going to take place?
Freddie: We’re going to rent an entire farm and set up a massive sound system that all the neighbors will be able to hear.
Mick: Wow! You’re really putting on a show.
Meaning: "You rock" is an expression used to show strong approval. Since Rock N’ Roll is a popular genre of music, it is an expression going back for some time. However, it’s still popular in use.
John: Hey man, do you think you can have that work done by the end of the day?
Paul: I completed that work last week. I can have it on your desk in a minute.
John: Fantastic! You rock! I can always count on you.
Paul: Thanks. I really appreciate your comment.
Meaning: In this instance, “You rock” is also used for approval, but it can also be used as a cheer from excitement.