Gravitational waves are distortions in the very fabric of space and time, usually created in collisions between black holes and/or neutron stars. The waves were first predicted a century ago by Einstein's theory of general relativity, but weren't directly detected until 2015, with dozens more picked up in the years since.
Although they originate from some of the most energetic events in the universe, by the time these waves reach Earth, they're actually only distorting reality on incredibly tiny scales – roughly one thousandth the width of a proton.
And that's just the big ones. All gravitational wave detections made so far have been sharp, sudden signals from cosmic cataclysms, but there also should be a steady background hum of lower-frequency waves out there too. But of course, those are much harder to detect, because they can take much longer to wash over Earth. But finding them is the goal of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav).