Electricity is supposed to come down one wire (the 'Live'...) and go back up the return wire (the 'Neutral'...). All is well with the world. Then comes the Incident, whereby a Fault somewhere allows the electricity to get to something other than the Neutral wire (a short-circuit, a breakdown in insulation, ingress of liquids, dog chewed a cable... The list is long...). In such a case, the sum of electricity going down the Neutral wire will be less than that arriving from the Live wire, the difference going, perhaps, through the body of a poor unsuspecting Musician, thanks to the Fault. This situation is what the 'Plug Thingy' (a technical term...) detects. Any imbalance above a certain 'safe' level between Live and Neutral is deemed to be potentially life-threatening (indeed, it is...), and the Thingy cuts the juice until the Fault is located and corrected.
There, that's the 'simple' explanation. Is it sufficient..?