Usually meaning the end of something, or a final part in something; that's the end.
Taken from the movie, Goodfellas during the scene where Tommy DeVito is supposed to be made a member of the Mafia.
*group enters an empty room*
Tommy DeVito: Aww no!
*Mob hitman shoots Tommy in the back of the head*
Mob hitman #1: And that's that.
Mob hitman #2: No fucking joke.
Mob hitman #1: Let's go, I'm fucking hungry.