From the mouth of the horse: "Wikidata is a free knowledge base that can be read and edited by humans and machines alike. It is for data what Wikimedia Commons is for media files: it centralizes access to and management of structured data, such as interwiki references and statistical information. Wikidata contains data in every language supported by the MediaWiki software." 
Wikidata interacts with MusicBrainz in various ways. This page is meant to note these various ways for MusicBrainz editors.
Interactions that affect MusicBrainz.
The most straight-forward are the Wikidata URL relationships. A few bots scour MusicBrainz and add Wikidata links to entities, but it can also be done manually.
There are several properties in Wikidata for MBIDs, all of which can be used to link an item on Wikidata to MusicBrainz:
- Area MBID
- Artist MBID
- Event MBID
- Genre MBID
- Instrument MBID
- Label MBID
- Place MBID
- Recording MBID
- Release MBID
- Release Group MBID
- Series MBID
- Work MBID
The template Authority Control on the English Wikipedia will automatically include a link to MusicBrainz.