The Contest Description on the contest page is simply an explanation of what you want freelancers to do. It is where you specify the fine points of your contest. The clearer and more detailed your contest description is, the better the chance you have of getting exactly what you are after in the shortest possible time.
When starting a contest, the details should 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 Contest Description will be posted under the Description tab for contest participants to review.
Related article: How Do I Write a Good Contest Description?