1) You don't care, or you don't know what to do.
2) When you really do care about
something but you don't want to hurt anyone's feelings, or make them mad or
something.
Boy: "So where do you want to
eat?"
Girl: (really wants to
eat at "McDonald's") "It doesn't matter".
Boy: "Alright, LET'S GO TO '
BURGER KING'!!!!"
Girl: (I was hoping that we could go to "McDonald's", but...ah, it's fine)