This app simulates how light rays refract through a spherical lens. Most notably the effect of spherical aberration can be seen. Basically, spherical lenses do not actually perfectly focus light and this is reflected by the fact that the light rays do not meet at a singular point right of the lens.
You can pan by dragging with your mouse, and zoom by scrolling with your mouse wheel. If you're on a touch device you can pan & zoom in the standard way.