Gravitational waves are a prediction of Albert Einstein's general theory of relativity, and involve the propagation of variations in the curvature of space-time. Gravitational waves, which propagate at the speed of light, were first detected in 2015, roughly 100 years after Einstein developed general relativity.
Gravitational waves are produced whenever an object with mass accelerates. Currently, detectable gravitational waves occur in the collision of two very massive objects, where the objects are much more massive than the Sun, e.g. neutron stars or black holes.