A-roll: the spine
A-roll is the footage that carries the words: an interview subject explaining something, a CEO speaking to camera, a customer giving a testimonial. If you watched only the a-roll with no other cuts, the story would still make sense.
B-roll: the visual layer
B-roll is everything that gets cut in over the a-roll: shots of the product being made, the team in motion, exteriors, environmental detail. It's what lets the editor cut around a stumble, condense a 4-minute answer into 30 seconds, and visually support what's being said.
Why the b-roll-to-a-roll ratio matters
If you shoot interviews but skip serious b-roll, you end up with a "talking heads" video that's hard to watch. If you shoot beautiful b-roll but no interviews, you have music-video atmosphere with nothing to say. The right ratio for documentary brand work is roughly 1 hour of interview to 3 to 4 hours of b-roll.
What good b-roll looks like
Specificity and motion. Don't shoot a static wide of the office. Shoot a designer's hands marking up a print, a manager scribbling on a whiteboard, the moment a machinist measures a finished part. The more specific the b-roll, the more it earns its cuts.