Why does thunder boom?

Thunder booms because lightning heats a narrow path of air almost instantly. The air swells outward so quickly that it sends a pressure wave through the sky. When that wave reaches your ears, you hear the sharp crack or deep rumble we call thunder.

The delay after the flash is a built-in measuring tape. Light arrives first, while sound lags behind, so counting the seconds tells you roughly how far away the strike was. A long rumble often means sound is reaching you from different parts of one crooked bolt.