Full answer
Inside a thunderstorm, ice crystals and water droplets collide and separate electrical charges — positive charges accumulate at the top of the cloud and negative charges build at the bottom. When the voltage difference becomes large enough, electricity discharges as a lightning bolt.
A lightning bolt can heat the surrounding air to about 30,000 Kelvin — roughly five times hotter than the surface of the Sun. This superheating happens in microseconds, causing the air to expand explosively.
That explosive expansion creates a shockwave that radiates outward as sound — what we hear as thunder. The rumbling quality happens because different parts of the lightning channel are at different distances from your ears, and the sound arrives at slightly different times.
You can estimate how far away a storm is using the "flash-to-bang" method: count the seconds between the lightning flash and the thunder, then divide by five to get the distance in miles (or divide by three for kilometers).