Plain English Meaning
A tiny gauge that tells the pilot whether the heading indicator is matched up with the airplane's magnetic sensor. Centered means matched. Off-center means the system is busy correcting itself.
Full Definition
A small instrument indicator, usually centered on zero with deflection markings on either side, that shows whether the remote indicating compass system is correctly aligned with the magnetic heading sensed by the flux valve. When the needle is centered, the heading indicator and the flux valve are in agreement; deflection to one side shows the system is in the process of slaving (re-aligning) the heading indicator to match the flux valve.