To unlock you must complete 50 days of the survival campaign, on day 50 you are rescued by the husband with the RV and drive away giving you an achievement and unlocking it. Defibulator, once charged can revive a member once. If your shelter is out of power the defib would not be charged. Once revived the member would be at half life and in a weakened state, while in the weakened state they cannot be revived again. Treat this as an extra chance at life and each member gets one.
Once a revived member dies again they will still have a defib countdown as if they can be revived, not sure if this is a bug or if it is intended (maybe in the future it would be unlimited) or maybe the dev wants you to watch your family pass away before you can harvest their flesh.