It means that it has to be true in order to fit the situation example. If some says .Lebron James is the best basketball pla<x>yer in the world and you say and he's super rich. I'd say that's a given because if you are the best pro ball pla<x>yer than you are going to be rich
Basically, It means it's obvious, but more like it's guaranteed.
Gotta hand it to you, you ask great questions...
Everyone with a brain knows it to be true.
It relates to gift giving at Christmas. It's a phrase Christians use, especially Catholics.