This is one of the best gravitational waves explainers we've seen

In the distant reaches of the cosmos, two black holes collide and merge in a violent crash. We can't see it or feel it, but the shock of that collision sends ripples propagating far and wide through the fabric of space and time. 
Those distortions, known as gravitational waves, affect every molecule they come across — warping their size by a fraction of a proton. However, those changes are only detectible by some of the most highly calibrated and sensitive tools ever built by humans.