Parallax wallpapers are a feature in iPhone 5's and iPad's iOS 7 and some Android devices that means a scrolling wallpaper / background image when you tilt your device. Some Android phones also have parallax wallpapers without motion-sensitivity.
"Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines." - Wikipedia: Parallax
Note that iPhone 4 does not support Parallax wallpapers even with iOS 7.