A standard definition of parallax: the effect whereby the position or direction of an object appears to differ when viewed from different positions, e.g. through the viewfinder and the lens of a camera.
In Extended Reality (XR) applications, Parallax is a 3D effect observed when images and other elements in the foreground of a screen move at a different rate than those in the background.
XR developers use parallax techniques to make the virtual world look more realistic. By adding in a parallax effect, the 3D world seems to behave much more like it would if the person were actually in that environment.