To defibrillate is to use a specialized machine that zaps electricity into a person's erratically-beating heart. Doctors and EMTs sometimes have to defibrillate patients having heart attacks.
The verb defibrillate comes from defibrillation and its roots, de-, "undo-, and fibrillation, "beating in an abnormal way." So when a patient's heartbeat is dangerously abnormal, doctors will sometimes defibrillate it to shock it back into a normal rhythm. The tool they use to perform this procedure is a defibrillator.