Secondary essay prompts vary, but there are a few that are standard fare. "Why do you want to attend our school?" is a common topic pre-meds will encounter.
Med school admissions officers want to be assured that you know their institution, are seriously considering it, and will fit in well there. In approaching the "why-our-school" question, do your research on the institution and link something specific about you with the school's philosophy, curriculum, surrounding patient population, and/or extracurricular programs.
For instance, if you were a teaching assistant for chemistry in college, you might link your use of the Socratic method with a school's tutorial-based learning. In that way, you demonstrate knowledge of the school, show that you connect well with it, and showcase your accomplishment.