Understanding cutaways, cut-ins, and the editing process
To understand what B-roll is, let us uncover where the term first came from.
In the film industry, the main camera team (the main unit) shoots the central action of the film’s key characters with Camera A, and a second camera team (called the second unit) will shoot other footage with Camera B, where you do not need the main actors present. The footage from Camera A is called A-roll, and the footage from Camera B is called B-roll. Both were named in the early days when movies were exclusively shot using rolls of film (Figure 8.1):
Figure 8.1: Image of rolls of film (LDGE at English Wikipedia): https://en.wikipedia.org/wiki/
Since B-roll from the second unit is used to capture elements of the story where the main actor is not needed, this frees up time for the main unit to focus on the main story, while the second unit captures other shots for the edit such as cut-ins or cutaways. Cut-ins...