Data quality levels help consumers of the data programmatically know what to expect of it, and allow MusicBrainz editors to find releases in need of a helping hand. It is not a mark of how good or bad the music in a release is - for that, use ratings.
Data quality levels
All available data has been added, if possible including cover art with liner info that proves it.
This is the default setting - technically "unknown" if the quality has never been modified, "normal" if it has.
The release needs serious fixes, or its existence is hard to prove (but it's not clearly fake).