Gravitational waves can be considered as ripples through space-time and are made by the motion of anything massive through space. when you throw a ball into a pond the ripples created will move away from the point of impact and bounce around the pond’s surface. These gravitational waves can be thought of similarly, but instead of rippling across a surface, they propagate at the speed of light through 3-dimensional space. They are theorized to be generated by the collisions of black holes and are thought to have been generated in abundance by the inflationary period just after the Big Bang around 13 billion years ago.
The Guardian offers some explanation of the idea which goes back to Albert Einstein here.
The concept of Gravitational Waves is part of the Inflation/Big Bang Theology and those who have seen this news as confirmation of their beliefs include Stephen Hawking.
And here's a video