A PRD, or product requirements document, explains what a product or feature should do and why it is useful. It is like a shared brief that helps a team agree on the goal before building.
A PRD for a new account page might describe who needs it, the important actions, and what counts as a successful result. It can give designers and developers a common reference.
A PRD is not necessarily a long document or a fixed design. It should provide enough clarity for the team to make good decisions, while leaving room to learn as the work progresses.