Free to Paid MRR
This is the monthly recurring revenue generated from subscriptions which are moved from free plans to paid plans. It also includes the revenue generated when subscriptions of paid plans move from a free period to a paid period.
If a subscription is moved from plan A (free) to plan B (MRR $150) then the free to paid MRR would be $150. Also, if a subscription is on trial in the previous period and is now on a paid plan B with an MRR of $150, then the free to paid MRR would be $150.