I'm certain I know which app this question is pulled from because I struggled with it a bit as well. I have personally decided to use the space to discuss an aspect of my life that would not fit into any other category on the application, but still had the potential to be related to my choice to pursue medicine. I get the impression you can pretty much discuss anything about your life/experiences that make you unique in some way, as long as you can, at the very least, tie in some tangential relationship to medicine.
At first I interpreted the question as a) optional and b) a place to discuss any felonies etc., until I noticed that asterisk staring up at me and sections elsewhere on the app to discuss any criminal offenses... I did speak with a current post bac at this school on the forums though (name is Evidence Based) and they were able to speak from their own experience. My real concern was discussing something that was irrelevant or "not what they're looking for" but the question seems to be purposely vague and open ended, allowing for an endless possibility of things to discuss.
I'm not sure if this was helpful, but I share your bafflement.