What is redshift and blueshift?
Suppose a star is rapidly approaching the earth. The light waves from the star will be compressed, or pushed together. In fact, wave lengths from an approaching star often appear shorter than they really are. Shorter wavelengths of light are characteristics of blue and violet light. So the entire spectrum of an approaching star appears to be shifted slightly toward the blue end of the spectrum, this is called the blue shift.
If a star is moving away from the earth, the light waves will be slightly expanded. The wavelengths of the light will appear longer than they really are. Longer wave lengths of light are characteristics of the red end in the spectrum. So the spectrum of the star moving away from the earth appears to be shifted slightly toward the red end, which is called the red shift.