red shift is a way of measuring if a light source (normally a star) is moving toward or away from you. The visible light gets shifted to the red end of the spectrum if the the 'star' is moving away from you (or is it the other way round?). Similarly, it is why a vehicle approaching makes a higher pitched noise than one moving away. You know the way a fast car goes eeeeeeeeeeeeee oorrrrrrrrrrrrrrrrrrrrrrrr sort of thing. All to do with the Doppler effect. Google 'Doppler effect' for tons of info.