Disinterested means "impartial" or "not taking sides." (In other words, not having a personal interest at stake.)
Uninterested means "not interested." (In other words, not showing any interest.)
Correct: A good referee should be disinterested.
(He does not take sides.)Incorrect: He was disinterested in Jill's hobby.
Correct: He was uninterested in Jill's hobby.
(He shows no interest.)