What does your app do? You should write an overview that explains what the app actually does. Writing your idea down will help to clarify it in your own mind. Not only will this then help you explain your idea to others, but it also forces you to be more detailed and helps you catch the points you may have missed.
Don't try to sell the app by using lots of exciting descriptions. Keep the language plain, simple, and easy to understand — Remember, not everyone reading this will be as informed as you are!
Write a detailed description of what your app does.