It is short for ground fault circuit interupter. Basically all current into a device should leave the device. If it is not then it is going to ground someplace else. (like through your rack.) If this outlet detects more current leaving than coming back it pops the circuit off. A ground wire is nice protection, but the current in the circuits hot leg has to increase to the rating of the breaker before it trips which is normally 15 or 20 Amps. If you have 5 Amps going to ground through your rack it still is not enough to trip the breaker and if you become a better conductor to ground for some reason like you are soaking wet and in contact with another grounded tool when you touch the rack that current can go through you. With the GFCI, if memory serves me right, if you get an imbalance of current between hot and neutral of 100 milli Amps or .1 Amps it pops off. Most current that can flow through you is 100 milli Amps with GFCI. With ground wire you could get as many Amps as it takes to trip the breaker before the circuit shuts off or worse you could get 5-10 Amps and be locked on (unable to let go) and still not be able to draw enough current to trip the breaker while you stand there and start cooking.