The total predictable revenue each month from subscriptions.
MRR is ARR divided by 12. It's the monthly version, what you'd collect this month if nothing changed. MRR is how most SaaS dashboards show revenue, because monthly changes are visible faster than annual ones. Track MRR, new MRR, expansion MRR, churned MRR, net new MRR, each tells you something different.
MRR is the pulse of a subscription business. Spiking MRR means acquisition is working. Rising churn MRR means something's wrong with the product. Healthy expansion MRR means customers love you. One number at 30,000 feet; three or four numbers at ground level.