What is Parallax?
You can think of motion parallax as what you see when you look out of a moving train. Bushes right alongside the train tracks will zip past your view, while cattle grazing in the field will appear to move more slowly across your view and the mountains in the background will hardly move at all. Parallax has particular relevance to astronomy. Not only because sci-fi movies use the effect all the time to create cool space effects, but because this effect is how we measure how far stars are away from our own solar system.
Check out the link above to see how it works.