The Doppler Effect is the change in wave length caused by motion. The classic example is the sirens on fire trucks and ambulances. The Doppler Effect is responsible for the change in the pitch of the siren as it speeds by. The way it works is the sirens are producing sound waves. When the truck moves, the distance between the waves is reduced, causing them to "bunch together" and travel at a higher frequency.
This concept can also be applied to water. The current in a pool or a pond usually travels at a steady rate. But when someone adds motion to this current, it causes the waves to move faster and closer together than they normally would. This is what causes the rings seen when energy is added to the water. The faster the motion, the closer together these waves become.