It's an examination, usually under general anesthesia, of the pharynx, larynx, trachea, esophagus with a rigid and flexible endoscope, sometimes with biopsies. I had several, one during direct biopsy as an outpatient, and another before a neck dissection. I had no ill effects, other than from the anesthesia, maybe a sore throat, if that, with the usual dietary, medicine restrictions, risks for any surgery, and someone has to pick you up or should, if its outpatient. It's also called a triple endoscope or triple scope, combining a Largynoscopy, esphoscopy, bronchoscopy.
Good luck.