After being placed on your farm, it won't do anything. When a storm happens, and lighting strikes in the valley, the rod will collect the energy. That energy is turned into a Battery Pack, the end product of the lightning rod.
How does it work?
Lightning Rod has about 80% chance (most cases) to be charged and gives you a battery on stormy day. They must be placed on your farm to be able to work. Lightning Rod takes about 24 hours in game to produce a battery.