It's an outdated attention-goosing technique that most designers will probably be embarrassed to look back on.

Parallax is the observed displacement of an object caused by the change of the observer's point of view. In astronomy, it is an irreplaceable tool for calculating distances of far away stars.

On iOS, the parallax effect is, of course, entirely simulated by the operating system’s software, with a little help from a technology that was originally conceived for rocket guidance.