B-roll (also B roll) is supplemental footage inserted into the main footage as a cutaway to help the story along. It’s particularly helpful during interviews to illustrate what’s being said and to break up the often visually boring talking-head footage.
The explanation to where the term B-roll comes from is a bit more involved. It stems from the use of 16 mm film editing where so-called A- and B-rolls alternated shots to allow for fades and dissolves between shoots. Somehow the term B-roll was later ‘abused’ for the conceivably less important support footage.