Because there are no additional fields in the online application for liabilities such as gaps in work experience or a low GPA, it would be wise to use this space to briefly cover any potential weak points in one’s candidacy. Such discussions should be direct and to the point, providing an explanation and pointing to mitigating factors without being defensive or making excuses. Mitigating one’s weakness or weaknesses here should be prioritized; however, given the open-ended nature of the question, there is also potential to discuss a particularly meaningful activity or project that could not be covered in the application materials. Determining a topic will depend on the information already covered elsewhere, so this may be worth revisiting after you have a sense of what you are able to share in the application process. Because this is an optional essay, a selected anecdote or additional information should enhance your candidacy and highlight a strong skill or special interest to stand out, as opposed to just be an extension of the short required essay.
Briefly state your position, state why the problem you are working on is important, and indicate the important questions that need to be answered; this is your "Introduction." Push quickly through this draft--don't worry about spelling, don't search for exactly the right word, don't hassle yourself with grammar, don't worry overmuch about sequence--that's why this is called a "rough draft." Deal with these during your revisions. The point of a rough draft is to get your ideas on paper. Once they are there, you can deal with the superficial (though very important) problems.