Title says it all.
If you want to explain something on your application (perhaps a family circumstance that led to a poor semester for example, or an extenuating circumstance that led to less course rigor). It’s not for excuses but simply for explanations. My advice is to ask your GC if you should put anything there.
I used mine to explain my blogging activity (which I didn’t list on my activities) and to further explain what a course was, since it was an intense seminar. It’s used to give more detail to colleges, if necessary. It is by no means mandatory.