It's an MD5 of a portion of the tune -- the first 64Kb past the ID3v2, the last 64Kb before the ID3v1, and the middle 64Kb. It is used to identify a particular track with a good degree of uniqueness, so that we can do proper duplicate checking.

IIRC, emplode doesn't do this yet (Rio Music Manager does), and JEmplode supports the same scheme, for future compatibility.