What a creative brief means
According to the Cambridge Dictionary, to brief someone is to give them detailed instructions or information, and a brief is a set of instructions or information. The word on its own covers much more than marketing: in law, for instance, a brief is the set of documents on a court case. That is why marketing and advertising use the longer name, creative brief, for the document that commissions a piece of communication, and that is the sense used here.
A creative brief has five parts. The objective says what should change: more requests for quotes, a specific group getting to know a product, existing customers buying again. The target audience says who the work is for, and a buyer persona helps describe that audience in terms of real goals and frustrations. The key message is the one thing that person should remember. The constraints cover budget, deadlines and mandatory content, such as legal notices or brand elements. And the measure of success states which metric the result will be judged by, for example a KPI agreed before work starts.
Two similar documents are worth keeping apart. The first is the counter-brief, the response from whoever receives the brief. In Spanish it is called contrabriefing, and Economipedia defines it as the document in which the agency corrects and improves the company's brief, pointing out missing information and advising the client. The brief sets the task; the counter-brief checks that both sides understand it the same way.
The second is the requirements specification of a technical project, in German the Lastenheft. The V-Modell XT Bund, the project model of the German federal administration, describes it as the document holding every binding requirement the client sets for the system to be developed, and makes it part of the contract. The contractor then works it out in its own specification, the Pflichtenheft, which the V-Modell calls the contractor-side counterpart of the requirements document. The relationship resembles that of brief and counter-brief, with one difference: a requirements specification fixes what a system must do, while a creative brief leaves the creative solution open.