Thunder and lightning happen at the same time. But light travels faster than sound. By calculating how much longer it takes to hear the sound of thunder than to see lightning it is associated with, this app calculates how far away the lightning is.
Just tap the app when you see lightning and tap again when you hear thunder.
The distance the lightning is away from you will be displayed in miles or kilometers.
Since the relationship of the speed of light and the speed of sound applies to all of physics, this app can also be used to tell how far away you are from a fireworks display.