Said aloud, 'a hundred' or 'one hundred' (often paired with the 100 emoji) is shorthand for 'totally agree' or 'true and serious'. It comes from the idea of giving something 100% — full marks, no doubt. 'You're right a hundred percent' becomes just 'a hundred'. If your teen says 'a hundred' as a reply, they are emphatically agreeing with whatever was just said. Informal, harmless, and very common in American youth speech now spread globally. No concern — it is a verbal thumbs-up.