What is a GFCI: The ground-fault circuit interrupter, or GFCI, is a fast-acting circuit breaker designed to shut off electric power in the event of a ground-fault within as little as 1/40 of a second. It works by comparing the amount of current going to and returning from equipment along the circuit conductors. The GFCI will “sense” the difference in the amount of electricity flowing into the circuit to that flowing out, even in amounts of current as small as 4 or 5 milliamps.
Purpose: A ground fault circuit interrupter can help prevent electrocution. If a person’s body starts to receive a shock, the GFCI senses this and cuts off the power before he/she can get injured.
Where should GFCIs be used: GFCIs are generally installed where electrical circuits may accidentally come into contact with water. They are most often found in kitchens, bath rooms and laundry rooms, or even out-doors or in the garage where electric power tools might be used.
How should a GFCI be tested: Whether you have a receptacle or circuit breaker GFCI, pushing the TEST button should turn off the power to the circuit. For the receptacle-type GFCI, pushing the TEST button should cause the RESET button to pop up. (Remember to push the RESET button to re-establish power and protection.) For the circuit breaker-type GFCI, pushing the TEST button should cause the handle to move to the tripped position. (Remember to reset the handle to re-establish power and protection.)
When should GFCIs be tested: GFCIs should be checked monthly to determine if they are operating properly. A portable GFCI should be used out-of-doors with various electrical power tools (i.e., drills, mowers, trimmers) and should be tested before each use!
Read more about GFCIs from InterNACHI here https://www.nachi.org/gfci.htm