It stands for Laser Interferometer Gravitational Wave Observatory!

The black holes are so big and fast when they spiral in and merge that they literally create waves in spacetime which change the length of things by a tiny amount as they pass by us and LIGO is able to measure when the two arms of it change length by nanometers and that’s where we got the signal.

There’s also The Event Horizon Telescope which made radio images of the black hole at the center of our galaxy. We haven’t been able to catch a merger with this though.