Room tone or air tone is the natural sounds present in the space where you are recording live audio for your video.
No environment is completely silent, no matter how quiet is may seem. Background elements such as buzzing lights, heating systems, distant traffic, etc in conjunction with the size and texture of the space give each space a distinct tone referred to as 'Room Tone'. 'Air Tone' is the same thing in external spaces.
Recording room tone is an extremely useful part of the filmmaking process, particularly when it comes to editing. It can help to fill gaps between action or dialogue and can help smooth out transitions between scenes. It can also be used to remove certain unwanted background audio elements when used with noise removal software.
Recording
Set up microphone(s) / recorders
Listen to your audio, set the levels appropriate to the space, the performers sounds, etc
'Quiet on Set' - Turnover - 'Rolling Room Tone ......'
Record for 30secs - 1min - Monitor the audio recording with headphones throughout
'Cut'
Repeat if there were any sound disruptions during the recording
Additional info:
Record for each microphone & position
Repeat if the microphone levels or positions are changed
This process is ideally done at the beginning of a shoot, but can be done at the end if forgotten.