Additional MRR from customers who had previously churned or canceled. Reactivation MRR is a component of expansion MRR. It is important for these customers to have contributed $0 to the MRR in the previous period (does not include users in free trials).
For example, if a subscription canceled in the previous month is reactivated in plan B (MRR $150) then the reactivation MRR would be $150.
Reactivation MRR can indicate that your customers felt their needs were better met by a competitor, but for various reasons, decide to come back. Or, it can mean they no longer had the need for your product or service but saw it resurface again.