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. The red vertical line left of the lens indicates (roughly) the spot where the outgoing rays will be parallel. Vice versa, the red vertical line right of the lens indicates the spot where parallel incoming rays will converge. The accuracy of these indicator lines depends on the thickness of the lens, if the thickness is small compared to the radi of the circles the approximation is good, this is called the thin lens approximation.
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.