Gravitational waves are thought to be ripples in the fabric of space-time, caused by violent processes.
From Davide Castelvecchi at Nature:
With a handful of discoveries already under their belts, gravitational-wave scientists have a long list of what they expect more data to bring, including insight into the origins of the Universe’s black holes; the extreme conditions inside neutron stars; a chronicle of how the Universe structured itself into galaxies; and the most-stringent tests yet of Albert Einstein’s general theory of relativity. Gravitational waves might even provide a window into what happened in the first few moments after the Big Bang. More.
The difficulty is, do gravitational waves exist or are they just today’s phlogiston – an idea that explained observations but did not represent the reality of chemistry? At least, if we are researching, we’ll find out.
See also: About ravens, we told you. They caused a mysterious glitch in LIGO data
Rob Sheldon on Physics Nobel for gravitational waves: Another PC moment in science?
At Forbes: Gravitational waves detection was all just noise, some researchers say