MediaWiki:Userrights-groups-help

You may alter the groups is in:
 * A checked box means the user is in that group.
 * An unchecked box means the user is not in that group.
 * A ٭ indicates that you cannot remove the group once you have added it, or vice versa.
 * Note that bureaucrat rights cannot be added once they are removed even if there is no ٭ to indicate this.
 * A # indicates that you can only put back the expiry time of this group membership; you cannot bring it forward.

More information on what each group means can be found at Project:User access levels.

A log of user right changes is available at Special:Log/rights.