This amazing scientific achievement was announced today, verifying Einsteins theory of relativity. We know gravity as the pull of the Earth; a steady dependable pull causing things to fall. There are events happening is space such as two black holes spinning into one another becoming one. The gravity field (pull) around this kind of event varies greatly with time. These variations are propagating away as a gravitational waves distorting the fabric of space. These waves can be detected with extremely sensitive instruments as now has been done. Here is a paragraph quoted from the announcement.
“You can almost visualize it as if you dropped a rock on the surface of a pond, and the ripple goes out,” says Nergis Malvalvala, the Curtis and Kathleen Marble Professor of Astrophysics at MIT. “[It’s] something that distorts the space time around it, and that distortion propagates outward and reaches us on Earth, hundreds of millions of light years later.”