Lightning is very hot. Hot air expands.

When lightning hea up the air inside itself and next to itself that airs expands a lot fast, because lightning is so hot. As it expands, it bumps into the not-heated-up air next to it. The sounds of the suddenly hot air bumping into cool air is thunder.

When humans hear thunder, sometimes we hear it as one big boom--a clap of thunder. Sometimes we hear it as a series of sounds unfolding--rolling thunder.

Whether you hear a given bit of thunder as a boom or a roll depends on where you are relative to the lightning that makes the thunder.

If they lightning is laid out length-wise relative to where you are, all the sound the lightning makes is equally far way from you and arrives basically at the same time. Boom.

If you are at one end of the lightning, the sound from the end close to you gets to you first, the next farthest away next, and so on down the the far end. Roll.