You're the man, with the article "the" shortened to da in an ebonic style.
Usually it shows admiration from the person who say it towards the person to whom the sentence is intended.
(person 1): I got that girl's phone number.
(person 2): You da man!
(person 1): Hey, can you lend me 10 bucks?
(person 2): Yes, here they are.
(parson 1): Oh man, thank you, you da man!