YouTube asks, 'Continue watching?' to protect your watch history and help make sure that you don't use Internet data if you're not actually watching YouTube.
This 'Video paused. Continue watching?' message will surface on your screen if you're watching videos with auto-play turned on before 30 minutes of uninterrupted watching on mobile, 60 minutes on web or 180 minutes on TV. If you'd like to continue watching, you can select 'Yes'.