Room Tone is the silent sound you record after a scene is shot in a particular room, or setting, to give the editor atmospheric silence to work with.
When you watch a movie, even if nothing is being said, or nothing really can be heard, there is at least “room tone”, because if there wasn’t, it would be a very jarring experience.
If you’ve ever been to an Anechoic Chamber, like the quiet room at the Bell Labs in New Jersey you know what I’m talking about. The anechoic chamber is the loneliest and eeriest experience even if you’re in the chamber with a bunch of other people. “Quiet” isn’t “no sound”.