Plain English Meaning
A sensor with no moving parts that uses two laser beams running in opposite directions inside a single solid block to detect how fast the aircraft is turning. Three of them together tell the aircraft systems exactly how it is rotating in all directions.
Full Definition
A solid-state rotation-sensing device built into a single block of glass-ceramic material, in which two laser beams travel in opposite directions around a closed triangular path. When the unit rotates, the two beams take slightly different times to complete the loop, and that timing difference is measured to determine the rate and direction of rotation about one axis. Three MRLGs mounted on perpendicular axes provide the rotation used by an (AHRS).