A rebrief is the agency or creative team playing the brief back in their own words to confirm they understood the task the same way the client intended it.
The rebrief is the cheapest quality gate in the entire creative process. It happens before any concept work, costs one meeting, and catches the misunderstandings that would otherwise surface at the first presentation.
A useful rebrief restates the objective, the audience, the proposition and the success criteria — and explicitly names the assumptions the team is making where the brief was silent.
If a rebrief produces surprises, the brief was not finished. That is a good outcome, discovered early.