Full explanation
Churn is the rate at which customers leave your product. A high churn means people sign up but quickly cancel, which makes growth almost impossible.
You can have great marketing and still fail if churn is too high. Fixing churn often matters more than getting more new users.
A healthy SaaS for small businesses usually has a monthly churn below 5%. Below 3% is excellent.
Concrete example
You start the month with 200 customers. By the end, 10 have canceled. Your monthly churn is 5%.