I got this song from the Twilight soundtrack. I'm not sure how it relates to this movie, but I loved this song.
I think this song means that this man had a dream where he was gone, and no one cared but one person. When he woke up, he thought about what he was leaving and who would care when he's gone.
In the chorus he says that when he's gone, he wants everyone to forget everything he's done wrong, and to help him leave reasons for people to miss him. He doesn't want anyone to hate him for what he's done, and for them to keep only the good memories of him.
This song is very moving and really has a meaning behind it, unlike many other songs these days.