What is red shift?

You see colours due to the length of a wave of light. When a distant galaxy is moving away from us the wavelength increases due to the Doppler Effect (which you may have studied using the sound of a passing vehicle - as the car moves away from us the sound it makes is lower, meaning a longer wavelength). As the galaxy moves away and the wavelength increases the light is shifted towards the red end of the light spectrum (ROYGBIV) and we see the galaxy look redder rather than white. Because many galaxies have this effect, in all directions from the Earth we can tell that the universe is expanding.