If the star is moving toward the Earth the light is compressed and shifts to slightly shorter wavelengths. If the star is moving away from the Earth the light is stretched and shifts to slightly longer wavelengths. This is like the doppler effect in sound. Sound moving away from the listener is lower in pitch and sound moving toward the listener is higher in pitch because the soundwaves are respectively stretched or compressed.

Light waves are bluer in colour when compressed and redder when stretched.