Here's why:

* Light and Wavelength: Light travels in waves, and the distance between two peaks of a wave is called its wavelength.

* Doppler Effect: The Doppler effect explains how the perceived frequency (and therefore wavelength) of a wave changes depending on whether the source is moving towards or away from the observer. Think of the sound of a siren: it sounds higher-pitched as it approaches and lower-pitched as it moves away.

* Redshift: When a star or galaxy is moving away from us, the light waves it emits get stretched out, increasing their wavelength. Since red light has a longer wavelength than blue light, this stretching causes the light to shift towards the red end of the spectrum, hence the term "redshift".

Important Note: Redshift is a key piece of evidence for the expansion of the universe. Astronomers observe that the light from most galaxies is redshifted, indicating that they are moving away from us, and the further away a galaxy is, the faster it's receding.