This is at the time where Tybalt stabs Mercutio. "Ask for me tomorrow, and you shall find me a grave man" means that Mercutio will be in a grave, meaning he'll be dead.
Mercutio's statement foreshadows the coming of his death, which results in Romeo becoming outraged (Tybalt just killed Mercutio, Romeo's good friend). This causes Romeo to kill Tybalt, which leads to his banishment, and so on.