Monthly recurring revenue is the income a subscription business can expect every month from its active subscriptions. Annual plans are counted as one twelfth of their price per month.
MRR leaves out one-off payments such as setup fees. It changes through new subscriptions, upgrades, downgrades and cancellations, and tracking those four separately shows where growth is really coming from.
Multiply it by twelve and you have annual recurring revenue (ARR), the same figure on a yearly scale.
Why it matters for a launch
It is the number indie makers share most often, because it says in one figure whether a product is working. For a launch, the useful version is smaller: how many of the people who came actually paid.