This is usually where you would include factors that impact your application that aren't anywhere else. For example - my tennis involvement/stats took what looked like an odd drop in my junior year because I had a severe injury at the end of my sophomore year. I used this prompt to explain that injury and how I overcame it with intensive physical therapy to get back into top form to play at the Academy.
Another example is a grade slip because of a family situation/injury/illness, but really, it is anything that you think will strengthen your application that didn't fit elsewhere in the application.