There was some discussion a while ago at How to explain Works
Basically it is a list of new tags but they are not currently used in Picard. My logic is that a recording may be linked to a work, and most commonly for classical music that work is part of another top level work, and it that case they represent the Movement and Work. However a recording may be linked to a work which is not part of another work, conversely the work may be part of a work which is part of another work and so on, in the current MusicBrainz database there were a few relationships that extended to six levels. In all cases the two most important works are the one linked directly to the recording which I call MUSICBRAINZ_WORK_COMPOSITION and the top level one called MUSICBRAINZ_WORK then MUSICBRAINZ_WORK_PART_LEVELN can be used for levels inbetween.
If a recording is linked to work that is not part of anything then both MUSICBRAINZ_WORK_COMPOSITION and MUSICBRAINZ_WORK will be set to the same work, this makes the logic for any script/app using these values easier.