We know the universe is expanding, and we have a pretty good idea of how fast it's expanding, but we don't know the rate exactly. That's because of the different methods we have to measure the rate of cosmic expansion keep giving us slightly different results. It's a nagging problem that bugs astronomers, so while they have worked to ensure current methods are accurate, they have also looked to new ways to measure cosmic expansion. One of these new ways involves gravitational waves.
Gravitational wave observatories such as Virgo and LIGO have gotten pretty good at detecting the mergers of binary black holes. They've observed nearly a hundred mergers, and from each merger, astronomers can calculate things such as the mass and rotation of each black hole. What's more difficult to determine is how distant the mergers were.