This is a big deal, a very big deal. The discovery of gravitational waves in spacetime is a monumental event in scientific history -- the culmination of over a century of thought, and several decades of failed experimentation.
According to the Guardian, to detect the waves scientists needed "a set of instruments so sensitive they could identify a distortion in spacetime a thousandth the diameter of one atomic nucleus across a four kilometer strip of laser beam and mirror." These instruments, called LIGO (Laser Interferometer Gravitational-wave Observatory ) detectors, picked up gravitational waves produced in the final moments of a black hole merger at 5:51 in the morning (US ET), and were picked up by both LIGO detectors in Louisiana and Washington.
The detection of the waves confirms a major prediction of Albert Einstein's general theory of relativity -- formulated over a 100 years ago in 1915. It is important because gravitational wave astronomy is, as The Atlantic points out, "fundamentally different than ordinary astronomy":
Most astronomy is based on light, which is an electromagnetic wave: a vibration in electric and magnetic fields. Gravitational waves are vibrations in the structure of space-time, which travel at the speed of light. Like sound, the frequency—the “tone”—of a gravitational wave often depends on the size of the system producing it. LIGO is particularly attuned to “high-pitched” waves made by pairs of black holes or pulsars right before they collide.
Here was the Washington Post on the history of the theory, and why gravitational waves have been tricky to nail down:
In the early 20th century, Albert Einstein finally came up with an explanation, and it's rather astonishing. First he grasped that gravity and acceleration are the same thing. His General Theory of Relativity, formulated in 1915, describes gravity as a consequence of the way mass curves "spacetime," the fabric of the universe. It's all geometry. Objects in motion will move through space and time on the path of least resistance. A planet will orbit a star not because it is connected to the star by some kind of invisible tether, but because the space is warped around the star....
One of the predictions of Einstein’s equations (though Einstein himself wasn’t ready to buy in fully) was the existence of gravitational waves – ripples in the spacetime fabric. Scientists in subsequent decades looked for such waves to no avail.
The University of Maryland physicist Joseph Weber built gravitational-wave detecting devices and claimed to have discovered such waves, but his claims were disputed and ultimately discredited. But there were physicists who were not ready to give up the quest, and they ultimately persuaded the National Science Foundation to fund the creation of LIGO, which has two facilities, one in Livingston, La., and the other in Hanford, Wash.
And now, LIGO has finally proven Einstein right, and scientists get to marvel at a totally new universe they thought was there, but could never actually see.