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.
 * A # indicates that you can only put back the expiration time of this group membership; you cannot bring it forward.