What is B roll for videos? From Wikipedia: "In film and television production, 'B-roll,' 'B roll,' 'B-reel,' or 'B reel' is supplemental or alternative footage intercut with the main shot. The term 'A-roll' referring to the main footage has fallen out of usage."
Your primary shot is your video subject, such as a person being interviewed.
Secondary shots or B roll are images that illustrate what the subject is talking about