When you see sudden bright flashes of light in the sky, you're seeing lightning, the release of electricity between clouds or the ground.
Lightning can be scary, and if it strikes a person, it can kill. You can tell how close the lightning is by how long the delay is between the light and the sound of the rolling roar or crack of thunder that accompanies it. If they're close together, get inside, quick! Because lighting strikes so fast, we use its name for other speedy things, like the lightning-fast service at a good coffee shop.