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.
Well the gravitational waves come from a mass that is moving. It's like electromagnetic waves are created by a moving charge. But because gravity is so weak you need it to be a very big mass moving very fast to be detectable. When black holes merge they spiral in and at the last moment they get to extremely high RPM with all that mass moving very fast.
Kind of an interesting thought but I don't know if it really counts to say that the mass and location of the black hole is really "information from inside the horizon" even though technically the center of mass is inside the horizon.