Paraphrased: Ask the applicant to explain any topic for 5 minutes to the one interviewing. Then the interviewer will draw a blank face. Talented applicants will ask the interviewer if he understands, less talented applicants will just continue their story.
(paraphrasing) Choose a topic and explain it thoroughly to me in 5 minutes. Impress me with your clarity and conciseness. Secret bonus challenges: Respond appropriately to my blank stares. Resist my tangential distractions.
"Pick a subject that you like and know well. You now have 5 minutes to explain it to me and make me understand what's the most important thing about it is. Go."