First discovered in 1913 by Georges Sagnac, it involves sending two beams of light in opposite directions around a closed path and then using a detector to examine the beams when they arrive back at the starting point.
If the light path, light source and detector is stationary then both beams will reach the detector at the same time. The beams will recombine constructively in a distinctive interference fringe pattern. If the apparatus is rotated with the two beams travelling in opposite directions, then the beams will travel different distances before recombining. The beam moving around the loop in the direction of the rotation will have a longer path, while the beam travelling in the opposite direction will have a shorter path.
See also: Gyroscope.