A gravitational wave is a time-dependent distortion in spacetime that carries energy from merging black holes.
General relativity predicts that accelerating massive objects can produce ripples in spacetime. Compact binaries are especially powerful sources because their strong gravitational fields change rapidly as the objects orbit and merge. The waves travel outward at the speed of light and stretch and compress distances as they pass.
LIGO made the first direct detection in 2015, observing a signal from two merging black holes. The measured waveform contained stages called inspiral, merger, and ringdown. Each stage carries information about the masses, spins, and final remnant.
Gravitational waves are not ordinary sound waves moving through air. They can travel through vacuum, and detectors measure extraordinarily small changes in the lengths of perpendicular arms. Their discovery opened a new way to study black holes that may emit little or no ordinary light.