A brief is a simple, but clear description of WHAT you will photograph, the MESSAGE or PURPOSE of the photos and HOW they'll be presented.
This can be just a few sentences, but needs to describe the 'what', the 'why' and the 'how'.
For example:-
I plan to photograph a group of friends in their teens at various social locations (including at home, at the park and at school). I want to show them laughing and talking and generally enjoying each others company. They will be dressed in brightly coloured clothing to show their bubbly personalities. I will be presenting my photos as high-quality photos that are suitable for printing.