LIGO is an interferometer. It fires a laser beam and splits it into two laser beams. Mirrors bounce them back towards a light detector and merge them. Normally, the two laser beams should cancel each other, so the light does not reach the detector, but any changes in space-time caused by gravity waves can change the laser beams, so that they don't cancel out fully. When this happens, the light detector will see some of the laser light, which it can then use to work out the size of the space-time distortion.