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 can place the user in that group, but it is not recommended. Be aware of what you are doing before you do something involving this.
 * A # indicates that you can only put back the expiration time of this group membership; you cannot bring it forward.