What does it mean if someone tells you they love you but are not in love with you?

It means he cares about you and loves you as a person, but is not "in love" with you. He doesn't have romantic feelings. Compare what you would say to your parents and what you would say to your boyfriend. You love your parents but you are not in love with them, he obviously adores you as a friend, but without having sexual or romantic feelings.

It is very easy to fall IN love with someone, but to love them you have to build a relationship and really truly care about them. After a while the couple probably doesn't stay in love anymore, but they love each other.