A briefing is the meeting between department management (hiring manager) and the recruiter to discuss the details of the position to be filled.
The aim is for the recruiter to grasp the requirements perfectly in order to be able to work flawlessly in terms of process and not to have to change the search again in the meantime.
Contents of a briefing are e.g. salary, benefits, work experience, personality, team structure, training and promotion opportunities, no-go lists, …
The briefing is necessary for the creation/editing of the job advertisement, sourcing, interviews and partly personnel development decisions.