Gravitational waves are emitted by astrophysical objects characterized by dense objects moving at high speed, such as black hole and neutron star binaries. The waves are deformations of space time that appear as oscillating fractional variations of length, as measured by the strain h = dl/l.
The (extremely small) variations in length, dl, can be measured using a Michelson interferometer. The interferometer's output signal, proportional to dl, will in general increase with increasing armlength l.