A locked wiki is identified by the icon. It signifies that the wiki (or wikis) cannot be accessed at all except by validated users. Basically it is your assurance that confidential content will be available only to those people who should have access to it.
An example of a set of locked wikis are the airline wikis. They are only accessible to people who have been validated to work for the airline. Of course, you still should not publish sensitive company content on Aviaddicts.