Light is formed of a band of colours called Spectrum. Violet colour has the shortest wavelength and lies at one end of the spectrum and Red Light has longest wavelength and lies at another end.
The movement of a star or galaxy affects its light as seen by the observer. When the star is approaching the observer the light shifts to the blue end of the spectrum and the light shifts to the red end, When the star recedes away.
This is known as the Doppler Effect or Doppler Shift. The Doppler shift of the galaxies show that they are receding and that the universe is in a state of rapid and accelerated rate of expansion. This astronomical observation is known as Red Shift.