Fail-secure refers to a locking mechanism that defaults to a secure or locked position when it experiences a failure or power loss. This is the opposite of fail-safe, which defaults to an open position when power is lost.
Fail-secure mechanisms are most useful when it comes to locking highly secure areas like server rooms. In the case of doors, a fail-secure lock should have a mechanical unlocking method to ensure that occupants can quickly and easily exit the room in the event of an emergency. They aren't as desirable when it comes to locking doors, as many fire safety codes require that doors automatically unlock to ensure that occupants can speedily evacuate the premises.
By employing a fail-secure approach, organizations and individuals can mitigate potential risks and maintain a higher level of protection for their assets, even in adverse circumstances where knowledgeable criminals might try to interrupt power to a facility.