A soundstage is a very large room at a film studio, built so that no outside noise can get in — no traffic, no aeroplanes, no rain on the roof. Inside it, film-makers put up whatever set they need: a kitchen, a spaceship, a street. Because the room is sealed and controlled, they can record the actors' voices cleanly while filming, instead of adding the sound afterwards. That control is the whole reason the place exists, and it is why so many scenes that look like they happen outdoors were actually shot indoors on a soundstage. The word belongs to the film and television industry and to people who read about it; it is not everyday conversation. It is countable, and it is also written 'sound stage'. Sound engineers use the same word for a completely different thing — the sense of space and position you hear in good stereo speakers — so context matters.