In this example, spacetime is perturbed by the motion of two black holes rotating around each other. Gravitational waves appear when some event perturbs the curvature of spacetime. We can think of a drop falling in a pond. It perturbs the water surface, and this perturbation propagates outwards in the water as a mechanical wave. In a similar way, when spacetime is perturbed, gravitational waves are generated and they propagate outwards at the speed of light (~300000 km/s).

Credits: Swinburne Astronomy Productions