Room Tone
Here's a bit of technical background. One of the things you need to record, when you're shooting on a soundstage, or for that matter, on location, is the noise that the place makes all by itself. The noise of the room. I know, it sounds silly, but each place has its own special sound, and it's important to record that sound so that later, when you have to fix an audio track, or record a wild line, you have something to put underneath it so that it blends in with the rest of the audio track that was recorded live...
