In
ENG (
electronic news gathering) work, b-roll refers to the extra
scrap footage that you
shoot that gives the
editor something to cut to when the main footage is unusable.
For example, say the mayor is opening a new building. Watching his talking head speak for 45 seconds would be boring. Plus, if you edit a longer speech down to 45 seconds, there would be a great many jump cuts in the footage. So, you fill up your b-roll with pictures of the exterior of the building, the crowd looking interested, etc. Then, later when you are editing, whenever you want to cover a jump cut or get bored of the mayor's face, you can switch the video to b-roll footage, while leaving the audio intact.
Effective use of a b-roll can also make it look like you shot an interview with multiple cameras, when in fact, you only had one.
This style of editing is why editing from two players to a single recorder is known as editing a b-roll.