Plain English Meaning
Small electronic sensors that detect which way is magnetic north, so the aircraft's instruments can show an accurate heading.
Full Definition
Magnetometers are sensors that measure the strength and direction of the Earth's magnetic field. In an Attitude and Heading Reference System (AHRS), they provide the magnetic heading reference that allows the system to display the aircraft's heading on electronic flight instruments.
Context Anchor
Seen in discussions of attitude and heading reference systems, especially where the system gets its heading information.