A creative brief is the document that defines a task for a design team: the business goal, the audience, the scope, the constraints and the criteria for success. The better the brief, the fewer rounds of revisions.
A brief describes the problem, not the solution. "We want more sales enquiries from the website" is a good starting point; "move the logo left and add a carousel" is not, because it closes the door on better ideas. A good design team will start with "why?" anyway.
A solid creative brief covers: the business goal (what should change and how you'll know), the audience, the scope and format of the work, constraints (budget, deadline, legal requirements), existing materials (identity, research, current copy) and the list of decision-makers. Two pages of specifics are worth more than twenty pages of generalities.
Why it matters for business: most "failed projects" are really failed briefs. Time invested in a good brief comes back as fewer revisions and a quote without a padding-for-uncertainty margin.