Plain English Meaning
A setup that keeps the heading indicator pointed correctly to magnetic north on its own, by quietly comparing the gyro's reading to a magnetic sensor in the wing and nudging it back into line whenever it drifts.
Full Definition
A system that automatically corrects a directional gyro by continuously aligning it with magnetic north using signals from a remote magnetic sensor (the flux detector or flux valve), typically mounted in a wingtip or tail away from magnetic interference. The slaving system combines the steady reference of a gyro with the magnetic accuracy of a compass, producing a indication that is stable and without requiring to manually reset it.