Basically, it has to do with the state of the security device in question when there is no power going to it.

If the door can be opened when the power has failed to a door strike or deadbolt, we call this "Fail Safe." This means that people can come and go as needed when there is no power energizing a door. Fail Safe dead bolts are important on egress doors where people can be expected to leave a building during an emergency, such as a power outage.

Fail Secure is the opposite. The door is secure and can't be operated when power is removed. Most door strikes are fail secure, meaning that the force that keeps the door from opening is in place when no power is going to the door strike. A keypad or relay release provides power to the door strike and the door can simply be pushed open. Of course, with a door strike, the door can still be unlocked and operated by turning the handle.

Use of Fail Secure deadbolts should carefully be considered when used on doors used for entry. Since there is a bolt extending into the door, when the power fails, the door can't be opened manually and there exists a potential for people to become trapped.