A fail-safe lock releases when power is removed, so the door opens during a power failure or fire alarm. That behavior suits life safety and is required on many egress doors. The cost is that cutting power also unlocks the door, which is why such doors rely on supervision and often on an alarm.
Fail-safe is a life safety decision before it is a security decision. The lock is designed to open when something goes wrong, which is exactly what an exit needs and what an asset-protection door does not.
Fail-safe in an emergency
Magnetic locks are the common fail-safe device: power holds the door locked, and removing power releases it. Fire alarm panels, power supplies and release buttons are wired to drop that power. The arrangement has to release on the fire alarm, on a manual release and on power loss, and stay released.
Where it is the wrong choice
A fail-safe lock on an unattended exterior door means anyone can enter by cutting power or by waiting for an outage. Where the door protects assets rather than lives, fail-secure hardware with free egress fits better. The behavior cannot be chosen by preference alone; code and risk both apply.