Doppler effect in physics is defined as the increase (or decrease) in the frequency of sound, light, or other waves when there is relative motion.

Doppler effect takes place when there is relative motion between the source and the observer.

If the source and the observer are moving towards from each other, the wavelength decreases. Because the decrease of the wavelength, blue shift is formed.

If the source and the observer are moving away from each other, the wavelength increasing. Because the increase of the wavelength, red shift is formed.