When an object that emits/reflects light moves towards another object, the light waves are compressed in a similar manner to how sound waves are compressed as a firetruck drives towards you. The sound of the firetruck has a higher pitch. Likewise, the pitch of the light that os being emmitted/reflected gets to a higher pitch the faster the objects travel against each other. When the 'pitch' gets high enough, the red light being reflected or emmited will be percieved as blue. This is known as blue shift. Likewise, the same objects moving away from each other will result in a lower 'pitch', progressing down into the reds and infrareds, known as red shift.