Prepaid MRR is the sum of all deferred revenue. It's the total recurring revenue you have collected from customers, but haven't provided the service for yet. Im short, its the money you'd have to pay back if all your annual subscribers would cancel and request a full refund for the months they haven't used yet.
It's also used to estimate the upcoming cost to serve annual subscribers.
Having customers in long-term plans is a good thing, so you'll want the prepaid MRR to be as high as possible