It means that the song does not have any profanity in the lyrics.
Some songs are available in two versions; the clean version and the explicit version. A song with offensive lyrics should always be marked as "explicit" so that you can avoid it if you wish. One can create a Smart Playlist in iTunes that specifically excludes explicit songs, which then makes that playlist safe to play in front of your parents, your children or anyone else who may be offended.
- So usually, the clean version would be marked as such, so that you know which version you're getting.
- Just because a song is marked as "clean" it does not definitely mean there is an explicit version.