B-Roll is additional footage used in an edit to support and expand on the story.
B-Roll is recorded during a broadcast news, documentary, or film production. It can also be stock footage. B-Roll does not always require sound to be recorded, but it is best practice always to record audio if needed.
B-roll should match what is being said in the audio of a sequence. "SAY DOG, SEE DOG" is a common phrase in broadcast journalism and documentary. For example, if we hear someone saying "dog" in the story, we need to see a dog in the story. Whatever is being said or discussed, the viewer needs to see it.
Here are a few more reasons to use B-roll:
Prevent boredom for the viewers by providing visual variety.
Hide continuity issues, like jump cuts, in the edit.
Introduce subjects or settings.
Clarify and give context to what’s being discussed.