MBID: Musicbrainz IDs.
An MBID is an identifier used by Musicbrainz to identify individual tracks on a disc, tape, LP, or other music distribution medium.
There is a 1-to-many correspondance between 'real' tracks and MBIDs, as translations and transliterations of a album and/or track are entered seperately in the Musicbrainz database. This may be changing in the future, but should be stable for the moment.