What I mean by this: information that is not self-contained in the song/album enhances or changes our interpretations of it. It can be something the artist said/did, the historical or cultural context surrounding the music, the inspiration for the song or its writing/recording process, or anything else that makes the target music more meaningful.
Arguably any music can benefit from knowing its "outside story", so I'm looking for particularly noteworthy examples -- ones where this information is especially significant. This is of course subjective, so really anything goes! If you think it's important, well, then that's good enough to share!