This song (to me) talks about past troubles that were never settled and wouldn't be "laid to rest". It could also mean that they were old friends but something tore them apart years ago and one of them never left it behind. My meaning (being in this situation at one time) is if you have something you haven't settled/never really have settled, just make it water under the bridge. To someone else, it could mean that you have been in an argument with someone and you both can't let it go (girl friend and boyfriend arguments, parents sometimes).