If you successfully matched a track to MusicBrainz and you have independently found the bar code for the release you can submit the pairing to the MusicBrainz database, this helps build the MusicBrainz Database. For example you may have matched a release to MusicBrainz but MusicBrainz does not contain the Bar code, however if you have the original release you can read it from the cover add it to the Bar code fields for the songs and then submit to MusicBrainz with this task.