Plain English Meaning
A small magnetic sensor mounted in a quiet part of the airframe that picks up which way is magnetic north and sends that signal to the heading indicator on the panel, so the gyro stays lined up with the real magnetic heading.
Full Definition
A remotely mounted sensing unit, typically installed in a wingtip or other location well away from electrical and ferrous interference, that detects the direction of the Earth's magnetic field and sends that heading information electrically to the heading indicator in the cockpit. It is the sensor element of a slaved gyro compass system, continuously correcting the directional gyro to keep it aligned with magnetic north.
Context Anchor
Seen in and heading indicator , usually as a component mounted away from and electrical equipment that could disturb its reading.