In film and video production, the term room tone means the sound of an empty room, or a room in which all the actors are standing silently.
To a newbie, room tone means silence. To an audio professional, room tone means the subtle, low-volume sounds present in every room. Importantly, room tones are not all the same — every room has its own unique sound.
Room tone is normally recorded in every room used in a film. Typically, at the end of shooting a scene, the cast will stand silent for a few moments while the room tone is recorded. It is desirable to replicate the scene's exact background noise when recording room tone. In theory this means all the actors are still present, lights are on and the camera is rolling. In reality this doesn't usually happen — the actors may leave the scene first, the camera is often turned off, etc.
Room tone is used as a sound bed to accompany any new sound that is added to a scene in post-production, for example, re-recorded dialogue. If the new audio is recorded in a separate studio, the difference in background noise would be subtle but different enough to ruin the continuity. Placing the room tone underneath any new dialogue prevents this from happening (naturally you also need to avoid adding any new background noise with the new audio).