Plain English Meaning
It's the automatic system that keeps the RMI's compass dial lined up with magnetic north on its own, so the pilot doesn't have to manually adjust it during flight.
Full Definition
A mechanism that automatically aligns the compass card of a Radio Magnetic Indicator (RMI) with magnetic north by linking it to a remote magnetic sensor (typically a flux detector or flux valve) mounted in a wingtip or other location away from magnetic interference. The slaving system continuously corrects the RMI's so that it always reads the 's actual , allowing the RMI's needles to accurately to the of or stations.