A short document explaining why a product is being built, for whom, which features go into the first version and how success will be judged.
PRD stands for product requirements document. It looks at the product from the user's and the business's side: what the problem is, who has it, the proposed solution, the features in priority order and the success indicators. It is shorter and less formal than a detailed specification and usually comes first: you agree on what and why, then spell out exactly how.
Two clear pages prevent weeks of building in the wrong direction. It is also what you show a partner, an investor or a team so they understand the product in five minutes. Product managers write it after many interviews and questions, which is what the Product Owner Noura does in Fanann: she asks, then writes the document and the task list and hands them to the builder. Practical tip: keep a not-now list next to the feature list. A first product succeeds by what you leave out as much as by what you put in.