Admissions representatives will be at tables arranged in alphabetical order. Representatives will describe features of their schools, discuss admissions criteria, and answer questions. They will also distribute materials about their schools. It is not necessary to dress up; school attire is fine.
Your time at Graduate School Day will be most effective if you prepare in advance of attending. Knowing basic information about the graduate schools, as well as what's important to you as you decide where to apply and ultimately where to attend, will help you decide what you need to learn from the representatives at the event.