The Ground Fault Circuit Interrupter is designed to protect people from severe or fatal electric shocks.
GFCI constantly monitors electricity flowing in a circuit, to sense any loss of current. If the current flowing through the circuit differs by a small amount from that returning, the GFCI quickly switches off power to that circuit. The GFCI wiring interrupts power very quickly to prevent a lethal dose of electricity. You may receive a shock, but you should not be electrocuted. Because a GFCI detects ground faults, it can also prevent some electrical fires.
All GFCI wiring should be tested once from time to time to make sure they are working properly and are protecting you from fatal shock. GFCIs should be tested after installation to make sure they are working properly and protecting the circuit.
Most of the time, GFCI wiring does nothing; it just monitors the difference in the current flowing into and out of a tool or appliance. But when that difference exceeds 5 milli amps, an indication that a ground fault may be occurring, the GFCI shuts off the flow in an instant — as little as .025 second.