The GFCI outlet can tell the difference between the flow of the current from a hot wire that flows through a neutral. If the difference is significant, the circuit is tripped by the ground fault.
The outlets are designed to protect electrical items as well as people who handle the items.
Do not use a GFCI for a washer or a refrigerator. These appliances have motor winding that can trip the circuit. GFCI outlets are meant for bathrooms, kitchens, garages and anywhere outside that you need an electrical outlet.
GFCI outlets are to be placed by sinks, bathtubs, hot tubs, spas, Jacuzzis or water sources in homes that need an electrical outlet within six feet of the source.
There is a line side and a load side to the GFCI outlets.
This means the incoming power is the line side and the outgoing power is the load side.
If this outlet is installed incorrectly, it will not work. The GFCI outlet has a test button and a reset button. If the outlet is working correctly, you can press the test button and the outlet will be shut down until you press the reset button.
The test button should be tested monthly to insure it is working correctly.