A closed wiki or locked wiki is a public wiki where editing has been disabled for everybody except for a limited number of users belonging to some local or global usergroups (usually stewards). The content wikis are locked when the respective projects are closed. (See Closing projects policy). Non-content wikis may be closed when they are not needed anymore. Any closed wiki is still available to read.
The lock (soft lock) is done by removing 'edit', 'move', 'upload' and other edit related userrights (see InitialiseSettings.php for a full list) from the '*', 'user', 'autoconfirmed' and 'sysop' local usergroups and then assigning these rights to the local 'steward' usergroup. So, among local users only stewards are able to edit closed wikis. Global users who have some or all removed userrights are also able to edit them. Currently only members of stewards, interface editors, system administrators, founder and staff global groups have such an ability.