Question

What is redshift?

Answer 1

As a source of light, such as a galaxy far away, recedes from the observer, the wavelengths of the light received become longer, toward the red for the optical ones.

Answer 2

it is doppler effect
when the thing producing a wave moves towards the observer it will result in the increase in the frequency of wave and when it moves away from the observer results in the decrease in frequencv
by formula
f = c/w
f is frequency
c speed of light
w is wavelength
c is constant
decrease in f results in increase in w
increase in f results in decrease in w
hence galaxies moving towards us have blue shift
and galaxies moves away from us have red shift

Answer 3

In more general terms, redshift is the apparent lengthening of the wavelength of light to an observer. It can be the result of relative motion, as mentioned above, or the result of light passing through or emerging from highly curved space-time (i.e. the result of light passing through or emerging from an intense gravitational field.

Answer 4

A third cause of redshift is the expansion of space-time itself (i.e. the expansion of the universe). The common component to these three different cases is that it's the apparent increase in the wavelength of light as seen by an observer. It's not an absolute change.