What is it about?
Parallax is a technique used to determine distances, as it is a difference in the apparent position of an object viewed along two different lines of sight. This difference is then measured by the angle of inclination between those lines.
This principle is for example used in astronomy, where scientists use it to measure larger distances (i. e. distance of a planet from Earth).
There are many types of parallax effects used in many different fields, one of them is the Parallax Scrolling, used in computer graphics.
Here, when background images move passing the camera slower than foreground images, we have an illusion of depth, even if it is a 2D scene.
This also means, that this effect describes how depth affects your perception of movements.
This effect could also be used in music to change the way the listener will perceive the song, creating depht and movement.
Here an example:
Wikipedia – Parallax
C. Prendergast – Parallax Scrolling: Examples and History
Wikipedia – Parallax scrolling