Note: This does not include subscriptions that upgraded from a 'free trial' to a 'paid plan'
Reactivation MRR = Sum of MRR from customers that previously churned
For instance, if a canceled subscription is reactivated in a pricing plan of 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. Every reactivation is a case study opportunity. You should try to proactively control this by asking customers why they cancel when they do. It will show you an angle of your product or service's value you probably missed.
If you observe your reactivation MRR rising, you may want to look into marketing or customer success campaigns targeted at bringing dead customers back to life. While this may be a good thing, it's worth looking into the cost of reactivation as well. Throwing in hefty discounts to get back customers who aren't a fit for your product, maybe a drain on your resources in the short run and raise churn in the long run.