GFCI stands for Ground Fault Circuit Interrupter. It constantly measures the current on the hot and the neutral wire of a circuit. If the current in one of the wires differs from the other by 1/10,000 of an ampere the breaker or outlet trips. It does so because that 1/10,000 of an ampere is leaking and flowing somewhere other than through the wire and that somewhere could be someone€s body. These are used in wet locations throughout the house. Kitchens, bathrooms, outside, Garages etc.