The Doppler Effect is the apparent change in frequency observed by an observer moving towards or away from a sound or light emitter. When the relative motion of an observer is towards the source a higher frequency is observed but when an observer is moving away from the source the frequency is lowered.
This is commonly experienced in the passing of a train where a bystander hears the pitch of the whistle change as it passes sounding like deee-daah.
In the case of light, when the observer and light source approach each other, the the wavelength of the light decreases, shifting the observed light to the blue end of the spectrum, known as a blue shift. Conversely, when the observer and light source move away from each other, the light shifts to the red end of the spectrum, known as a red shift.
The effect was named after Austrian Christian Johann Doppler, who provided the first technical explanation of the phenomenon in 1842.