Redshift is a measure of how fast a celestial object is moving relative to us. If you have ever stood by the side of the road as a car passed by, you have a sense for what redshift is. As the car moves toward you, the pitch of its engine sounds higher than the engine of a stationary car. As the car moves away from you, the engine's pitch is lower than for the engine of a stationary car. The reason for this change is the Doppler effect, named for its discoverer, Austrian physicist Christian Doppler. As the car moves toward you, the sound waves that carry the sound of its engine are compressed. As the car moves away from you, these sound waves are stretched.