Doppler redshift is a phenomenon that occurs when a light source moves away from an observer. As the source moves, the wavelengths of the light stretch, causing the light to shift toward the red end of the spectrum. This effect is commonly observed in distant galaxies, indicating that they are moving away from us. This redshift is a key piece of evidence for the expansion of the universe. By measuring the Doppler redshift of light from celestial objects, astronomers can determine their speed and distance, helping to map the structure and evolution of the universe, including the work of scientists like Edwin Hubble.