Full explanation
MRR is the total amount of money you can count on each month from paying customers. It only includes recurring payments, not one-time sales.
It is the most important metric for any SaaS or subscription business. It tells you how healthy your revenue base is and how fast it is growing.
To calculate it, add up what every active subscriber pays you per month.
Concrete example
If you sell a subscription at 10€ per month and 100 people pay, your MRR is 1 000€. If someone cancels next month, your MRR drops to 990€.