The term dates to physical film, where the A-roll carried the primary footage and a second B-roll carried alternate material spliced in. The name outlived the workflow. Today it means everything that is not the main shot: establishing views, cutaways, detail inserts, hands working, environment.
It solves an editing problem as much as a visual one. Cutting between two takes of the same interview creates a jump cut; laying b-roll over the seam hides it completely and lets you compress a rambling answer into a tight one. Without b-roll, the edit is stuck with whatever the subject actually said, in order.
The usual failure is treating it as an afterthought and shooting generic filler. Strong b-roll is motivated — it shows the thing being described at the moment it is described. The practical habit is to shoot far more than seems necessary, hold each shot noticeably longer than feels comfortable, and vary focal lengths so the cutaways do not all look identical.