A system of massive moving bodies loses energy in Gravitational Radiation (G.R. in short), as described by general relativity theory, if the acceleration of the quadrupole moment of the system changes in time. The quadrupole moment measures the asymmetry of the systems mass distribution in space.
Einstein's prediction of gravitational radiation is generally considered to have been confirmed by the measured frequency decrease of the double pulsar PSR 1913+16. Sadly, this collapsing binary source is to weak to be detected in a direct measurement of G.R. on earth.
However, possibly measurable other sources of G.R. have been identified too, notably star collisions, supernova's and black holes. A supernova, that is a stellar collapse, though it releases enough energy to be measurable, is such an extremely rare event in our own galaxy - supposedly one every few decades - that one could hardly sit and wait for it. So we to turn to supernovae from other galaxies. We may expect even one event per month from galaxies up to the Virgo cluster at 10**7 parsec, and thus we have to adapt the sensistivity of the detector to measure those much weaker events.