How does an energy pyramid to
describe energy flow in a food web?

They help to describe the energy flow in a food web because of the transfers of energy between levels.

At the bottom of the pyramid you have technically the most energy. These would be things like grass, bugs, or even plankton and small cells if you want to go down that low. But these things individually don’t contain a lot of energy but put together they contain the most energy in the pyramid because there’s so many of them. As you go up the pyramid the energy in each organism becomes more and more concentrated, though there’s less and less of them. On average there is only about a 10% energy transfer from a lower level to an upper level. So the most energy would be on the bottom, but spread apart, and the least energy would be on the top, but very concentrated.