That is, whatever past Foo did is done. Maybe he was a dickhead and didn't study for the exam that future Foo is gonna have to sit. So present Foo can be a better friend and get that shit out of the way now, so future Foo can relax, pass, and gets to drink beer.
Like that concept of being your own best friend, only the friend you're forgiving is your past self and the friend you're doing favours to is future you.
Helps to get away from hang-ups about what is already in the past, and for some reason many people struggle with the idea of themselves as a person they should be kind towards.