1. What a comeback can mean a few things. I would say that the main usage and main definition is to go from way behind in a race (or something else), a game, even life to the place where that person is far FAR ahead of where they started from in the game, life, etc...

2. What a comeback can also be said when someone else is slamming or dissen another person, and the person comes back with a witty, smart, hard slammen comeback that tops the other person's cut down so badly that they are put to shame. Sometimes you get to watch a fight after you can say: Man! what a comeback! It aggravates the person who got slammed by the comeback.

When you eat dirt for the first time and leave your family to build a shack in the woods