In A Very Short Introduction: Black Holes by Katherine Blundell, the author discusses the merger of two black holes in a binary system:
The energy released in the merger of two supermassive black holes in a binary system is staggering, potentially more than all the light in all the stars in the visible Universe. Most of this energy is dumped into gravitational waves, ripples in the curvature of spacetime, which propagate across the Universe at the speed of light. The hunt is on for evidence of these waves.
What's exactly meant by that?