Final answer:
A redshift in light is observed when a light source, such as a galaxy, is moving away from the observer, and is a result of the Doppler effect.
Step-by-step explanation:
A redshift for light indicates that the light source is moving away from you. The phenomenon of redshift is a result of the Doppler effect, where the wavelength of light extends as an object moves away from the observer, causing the light to appear redder. Conversely, when a light source moves towards us, the light undergoes a blueshift, meaning its wavelength shortens and the light appears bluer. An example would be observing galaxies in the universe; as they move away from Earth due to cosmic expansion, their light is redshifted. This is a cornerstone observation supporting the Big Bang theory.