B RollB roll is the secondary or "safety" footage for a film. In order to string together two interview clips that were not shot consecutively, an editor will cut away from A Roll to B Roll, while the audio from the A Roll shot plays under. Then when the editor cuts back to the second A Roll shot, it appears as if the concepts were always married together.
This technique of using the cutaway is common to hide zooms in documentary films: the visuals may cut away to B roll footage of what the person is talking about while the A camera zooms in, then cut back after the zoom is complete. The cutaway to B roll footage can also be used to hide verbal or physical tics that the editor and/or director finds distracting: with the audio separate from the video, the filmmakers are freer to excise "uh"s, sniffs, coughs, and so forth. In fiction film, the technique can be used to indicate simultaneous action or flashbacks, usually increasing tension or revealing information.