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.