"Parallax, or more accurately motion parallax (Greek: παραλλαγή (parallagé) = alteration) is the change of angular position of two stationary points relative to each other as seen by an observer, caused by the motion of an observer. Simply put, it is the apparent shift of an object against a background caused by a change in observer position."
Its like on a cheap red dot scope where if you move the red dot isn't on teh zero you put it at. Aim your finger at an object about 15 yards away now move your head to the left or right while not moving and staring at your finger, notice how its not