Question

What is red shift?

Answer 1

Simplest Answer: the displacement of spectral lines toward longer wavelengths

Explanation:

Definition: Red Shift is a phenomenon connected with light emitted by a source in movement.

More details: Consider a source of light of wavelength `lambda_0`;
If the source and the observer are at rest then the observer will detect a wavelength `lambda_0`:

If now the observer is at rest but the source is moving (away from him) with velocity `v` then the observer will detect a longer wavelength `lambda>lambda_0`:

You can think of it remembering that:
wavelength = velocity x time
In the first case you have: `lambda_0=ct` where `c` is the speed of light;
But in the second case you have velocity=`c+v` that include the contribution of the source velocity and so gives you a longer wavelength and consequently a shift towards the red (longer wavelength) of the detected light!!!

In normal life (for example, looking at tail lights from a car) you do not detect this phenomenon because the velocity is not very big to influence significantly the detected colour. In case of fast moving object (in space for example) this phenomenon becomes observable.

(hope it is not confusing)