Redshift is the name given to the change in colour of objects which are moving away from us. At normal speeds this change is so small we do not notice, but when we use powerful telescopes to look at distant galaxies the colour change is measurable.
The cause of redshift is that light is a wave, and when the source of light moves away these waves are stretched out to a longer wavelength and so change colour towards the red end of the optical spectrum. Likewise, when objects move towards us their light waves are compressed and they look bluer than stationary objects.
We observe a similar phenomenon in our everyday lives when a vehicle passes us and we hear the sound of its engine drop as its sound waves change from being compressed (higher pitch) to stretched out (lower pitch). In this case the effect is called the “Doppler shift”
Astronomers use a value called z to measure the redshift of distant objects. It is calculated as the fractional change of wavelength of the moving object relative to a stationary object. Redshifts have positive values, blueshifts have negative ones and the z value of a stationary object is zero. The larger the z number, the faster it is moving.