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)