A ring laser gyro is an equilateral triangle and along each side it shines light from one corner to the other.
As the gyro accelerates in different directions, the wavelength of light appears to be stretched or compressed. The amount of this is measured by the aforementioned sensors, and computed into a position.
The compresssion of the wavelengths of light is known as doppler effect. Sitting in a train, the bells at a crossing are high pitched as you approach - the wavelengths are getting compressed from your viewpoint and appear to sound higher in pitch. Conversely when you are past the crossing, the pitch deepens.