Unless you have an extreme circumstance, I wouldn't put anything in that box. It's there in case there are any special situations they need to take into account when evaluating your application...i.e. if you had an immediate family member pass away/get very sick and you failed a class as a result, or if you had a personal injury/illness that prevented you from doing well or made you temporarily withdrawal. If you didn't do well in a course just because you were taking a lot of classes at that time, or for something less extreme than illness/injury, I wouldn't necessarily write anything there - don't use it to allow yourself to make excuses unless it's a truly legit thing.
It can also be used to explain things further if something needs clarification...i.e. my university didn't offer a regular human physiology course, so I did it as an independent study. On my transcript it said: "Bio 399: Independent Study", so I used that box to explain that that course was actually a human physiology course so that they would know to count it as a prerequisite course.
