A Release group is used to group several different releases of an album into a single entity.
Albums are often released in different ways in different markets. For example one album could be released in the USA with 14 tracks and re-released in Europe with one additional bonus track. In a colloquial sense they are the same album but not exactly the same to MusicBrainz. A release group will group together these two slightly different releases of the same album.
When an artist says "I have released my new album", she's talking about a release group. To her it does not matter much whether the USA version will be exactly the same as the European version.
When her publisher says: "Today we released her new album, next week we will released a limited edition version with two bonus track", he is talking about different releases of the release group the artist has told you before. Both these version will be grouped in the same release group.
Additionally, MusicBrainz considers each disc in a disc set a different release. Release groups are also used to group these releases into a single entity.
MusicBrainz has a set of guidelines on when and how to group releases together in a release group.