From b2evolution manual

Starting with version 2.0, Users Levels control which users can edit content created by other users. Users with a higher level can be given permissions to edit posts by users of lower level, or of equal level. (This requires to enable advanced permissions on the blogs where you want that to happen).

In previous versions, users levels would also allow to control access to some features, but this is now controlled by the User permissions.

- DEPRECTAED - In previous versions here is what user levels would allow:

• Level 1 and above:
  • Ability to view link to admin section (users with level 0 can still login into admin, but there's no good reason for that, except in rare cases, so they'll have to find out by themselves...) (on versions < 0.9.2).
  • Ability to upload images (on versions < 0.9.2).

• Level 2 and above:
  • Ability to generate static pages (on versions < 2.0 -- In 2.0 ability to generate static pages is granted to anyone with any post permission.).

• Level 5 and above:
  • Ability to edit timestamp on posts and comments.