A social security disability or SSI hearing is where you go before an administration law judge who's going to hear your case. I tell my own clients this is your best opportunity to win your case, as statistically, this is the most likely place where a claim is approved. As far as what happens at a hearing, the judge will ask you a series of questions about what your limitations are. The judge should have viewed your medical records, and will be very familiar with your medical conditions, but the judge really wants to know how those conditions impact and impair you on a daily basis. Get a feel for what it is about your condition that keeps you from working full time.
Your attorney or representative will then have an opportunity to supplement and ask you additional questions about your impairments. If there's a vocational expert in the room, then the judge may ask the vocational expert some questions about what types of work that may be available to someone like you with your types of medical problems. Your attorney will also have a chance to ask some additional questions of the vocational expert, as well. Finally, if there's a medical expert, then the judge may ask the medical expert or doctor some additional questions about whether you meet the medical listings of impairment, and your attorney will have an opportunity to cross examine them as well.