Ever since Isaac Newton in the 17th century, scientists had believed that gravity is a force that acts immediately on an object. For example, planets in our solar system move in straight lines except when gravity pulls them into curved orbits.
In 1915, Albert Einstein redefined gravity in his theory of General Relativity. He said that gravity isn't a "force" at all, but a curvature in space. The presence of mass or energy doesn't affect objects on earth directly; it warps the space around it first, and the objects move in the curved space. The earth always travels in a straight line. The presence of the sun curves space and thus the earth appears to be moving in an elliptical orbit. He also said that the gravity produced by a large object's mass would cause light to bend as it passed nearby.
Gravitational waves are "ripples" in the curvature of space and time, made by the motion of matter, much like a pebble tossed into a pond will cause waves to ripple outward from the source. But gravity's "waves" are not traveling through space-time, like light or sound waves; rather, it is space-time itself that is rippling up and down. By measuring how far apart these waves occur (their frequency) and how those frequencies change, scientists can deduce the source and shape of the object that produced them.
Unlike light waves, which bounce off material objects and enable us to "see," gravitational waves are very difficult to detect. They are too weak to be changed whenever they pass through some object in space. So gravitational waves carry signals unaltered during their journey across the universe. Detecting them would allow scientists to confirm that gravity travels at the speed of light, as well as the existence of black holes, and perhaps give them a clearer "snapshot" of the birth of the universe itself.