A gravitational wave is a ripple in spacetime itself, produced whenever massive objects accelerate in just the right way, most powerfully when two black holes or two neutron stars spiral into each other and merge. As the wave travels outward, it stretches and squeezes space by an almost immeasurably tiny amount.
Einstein predicted their existence in 1916 as a consequence of general relativity, but he doubted they would ever actually be detected, since the effect on any real experiment looked far too small to measure. That changed a century later.
In 2015, the LIGO detectors directly measured a gravitational wave for the first time, confirming Einstein’s prediction and opening an entirely new way of observing the universe, one based on spacetime itself rather than light. Astrinova’s piece on the experiments that proved general relativity covers this in more detail.
LIGO detects gravitational waves using laser interferometers, splitting a laser beam down two perpendicular four kilometer tunnels and measuring whether a passing wave stretches one arm relative to the other by a distance thousands of times smaller than a proton. The first detected signal, in September 2015 and nicknamed GW150914, came from two black holes, roughly 29 and 36 times the mass of the sun, merging more than a billion light-years away. Since then, gravitational wave astronomy has opened the door to multi-messenger observations, most famously in 2017, when a neutron star merger was caught simultaneously in gravitational waves and light, letting astronomers watch and listen to the same cosmic event through two completely different channels for the first time.
Ground based detectors like LIGO can only catch gravitational waves at certain frequencies, so a space based mission called LISA, planned by the European Space Agency for the 2030s, aims to fly three spacecraft in a triangular formation millions of kilometers apart, sensitive to slower, lower frequency waves produced by merging supermassive black holes that ground based instruments simply cannot detect.