A creative brief is a document outlining the goals, audience, constraints and context for a design project — written before concept work begins, to make sure client and designer are aligned on the problem before anyone starts designing a solution.
A strong brief describes the business problem and desired outcome rather than prescribing a specific visual solution, leaving room for the designer's expertise to determine the "how" once the "what" and "why" are clear.
Some designers write the brief themselves based on a discovery conversation; others ask the client to draft an initial version. Either way, a documented brief — rather than a purely verbal conversation — gives both sides a clear reference point throughout the project.