A safety device that shuts off power when a change in current is detected. This can protect life and property so knowing how they work and where to install them is important.
The GFCI is based on Kirchoffs current law that states: Any currents flowing into a junction must equal the sum of currents flowing out of that junction in a parallel circuit. Simply put current going in = current going out.
Using this principle a GFCI monitors current in the hot (black) wire and the neutral (white) and the current in both should be equal according to kirchoff's law. If there is a leak of between 4-5mA this would shut down power. This leak can occur due to short circuits which means current takes an unintended path either through faulty wiring, malfunction or going through a human body.
If a GFCI trips it is required to be manually reset at the receptacle, it is also important to use the test button on your GFCI once a month to ensure it is working properly.