MRR stands for Monthly Recurring Revenue. This stat is the amount of money generated by subscriptions.

The Math:

(Total Active Subscribers) x (Price of Subscription) = Total Monthly Recurring Revenue

Examples:

If you have 100 monthly subscribers paying $97/mo., your MRR would be $9,700/mo.

If you have 100 annual subscribers paying $970/yr., your MRR would be $8,083.33/mo.
(annual $97,000 / 12 months)

What It Means:

MRR is essentially the foundation of revenue that you generate on an average month. MRR is primarily used as a valuation metric for the health of subscription and software companies. 

MRR is not the same as Cashflow. Cashflow is the actual projected revenue, whereas MRR is the average monthly revenue.