MediaWiki:Userrights-groups-help

You may alter the groups this user 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.

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

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