Light emitted by a star is a spectrum consisting of many colors. If the star approaches the Earth, the shift of spectral lines of the spectrum takes place towards the violet end of the spectrum. This is called violet or blue shift. It indicates the decrease of wavelength or increase in frequency.
If the star moves away from Earth, the shift of the spectral lines takes place towards the red end of the spectrum. This is called red shift. It indicates increase in wavelength or decrease in frequency.