A brief is when you give instructions to someone or inform them, saying what they have to do for the task that has been set. A brief can either be written down or given verbally to whomever it is that needs the information.
It also gives the deadlines that are set so that they know when the work has to be handed in. A brief is important because it is the only way that you can inform someone of what it is that you want from them and when.
With a music video a brief could be the times that you need the band or artist for filming and what the storyline is going to be so that they know what is going on in their music video, as after all it is a video that is being produced for them.
The brief does have to follow some laws so that they don’t exploit the band or the artist that has put trust in you. In the brief you would comment on anything inappropriate, in example if a racial comment or remark is in the video or even implied and the band didn’t know anything about it and it is offensive to them then they could sue the maker.
Also anything incriminating in the video could give the band a bad reputation that they might not have wanted. One opportunity that a brief could present is any of the idea the band have that they can bring forward so that they can have a say in what is in the video. It also helps in organising everyone so that cast and crew know what it is that they are doing and when.