The Pyramid Principle is an approach to getting your message across by starting with the main idea supported by the relevant facts or arguments grouped by linking ideas. In this way, the writer clearly makes their point and shows how each idea supports it rather than having the reader try to piece it together as they muddle through.*
As everyone is different, we can't help others draw inferences different than we intend if we present each point in isolation. Instead, leading the reader through by first showing where you're going then raising and answering questions in turn, helps make the reader's life easier.
Barbara Minto, who presented this idea in her book The Pyramid Principle, distinguishes between our everyday approach of thinking through an issue where we often start bottom-up and ultimately get to, or figure out, our main point at the end. While this may be fine for a memo to reschedule a meeting, it's not ideal for an important proposal. Communicating bottom-up makes the reader work harder throughout and risks losing them along the way. Minto advocates doing the work upfront of organising your ideas and presenting them top-down, in a pyramid fashion, to get to the outcomes you want.
Rather confusingly, a related writing technique for journalism is called inverted pyramid writing.
*I first wrote the last sentence here in reverse while thinking through my point: "In this way, rather than having the reader wonder as they go through what point you're making and how the ideas fit together, you have instead done this work for them." Interesting to compare the two.