Well I think it's about a girl who never realized she love this boy until it's too late. I'm not saying the boy died, but maybe. Something happened and she never managed to "pay him back" everything she "owed" him. Or probably she never got to thank him until the time she sang this song. The first time I heard it I honestly thought that Haruno Sakura should sing this for Uzumaki Naruto. It's really a beautiful song, but quite sad too.