Here on Earth, we associate lightning with rain. On Mars, there is almost no water, and what water is there is frozen in the polar regions. The lightning found on Mars does not form in rain clouds, but dust clouds. We don't have a very good understanding of how lightning forms here on Earth, and even less of a understanding of how it works in these dust clouds on Mars.
Mars is an amazing world and like everything in our universe, continues to defy what we think is possible. It was just three years ago a group of scientists said that there was probably no lightning on Mars. Maybe studying this Martian lightning will help us better understand how lightning here on our own world works. Maybe Spirit can image some of this lightning on its overnight viewing session (unlikely though).
Source- University of Michigan