Why do we hear thunder after seeing lightning?
The short answer
Lightning and thunder come from the same electrical discharge and happen almost together. We see the flash first because light travels much faster than sound.
How thunder is made
The air along a lightning channel is heated extremely quickly. It expands suddenly and then contracts, sending a strong change in air pressure through the atmosphere. That pressure wave is thunder.
The lightning channel can stretch a long way through the sky. Sound from the nearest parts arrives first, while sound from farther parts arrives later. That is why thunder may sound like a short crack or a long rumble.
What the delay tells us
A longer delay generally means the lightning is farther away. Do not stay outside to measure it, though. If you can hear thunder, move into a substantial building or an enclosed vehicle because the storm is close enough to be dangerous.