A design brief, tagged as Contest Description on the contest page, is simply an explanation of what you want your freelancers to do. It is where you specify the fine points of your contest. The clearer and more detailed your design brief is, the better the chance you have of getting exactly what you are after in the shortest possible time.
The design brief is really just the description you input in the relevant field when starting a contest. Details include the font type you want, the colors you wish for it to have, the theme you want to be implemented, the purpose you have for it, etc.
It is advisable to be as thorough as you can so freelancers can have a better sense of what you require of them.
Once your contest is posted, the design brief will be shown under Contest Brief for designers to review.