Whenever you check out a concert or a play, you sit in an auditorium — that is, the section of a theater or concert hall reserved for the audience. You can try sitting on the stage, but you'll probably be thrown out.
Sometimes auditorium refers to the whole building where performances take place, not just the area for the audience. As the spelling of the word suggests, it's related to the word auditory, meaning "something related to hearing." In fact, auditorium is a Latin term meaning "a place where something is heard," — and of course, as high school and college students know all too well, that can mean lectures as well as concerts or plays.
