What does tu es mu bueno mean?

This is improper Spanish.

The statemwent "Tú eres muy bueno" by itself addressing a man or boy means "You are a very good person". The same statement can also mean "you are very good at..."

"[Tú] eres muy bueno en el béisbol" = You are very good at Baseball.

"Tu es mu bueno" literally translates to "Your is moo good"