Churn rate represents the amount of users that do not renew their subscription to a periodically charged service, such as SaaS or similar. Churn rate is easily calculated, you take the amount of subscribers you had at the beginning of a set time frame, lets say one month, and take the number of subscribers lost during that period divided by the numbers you started with.
For example if you start the month with 10,000 subscribers to your email marketing campaign and you lose 300 of those original 10,000 your churn rate would equal 3%.
It is important to only count the subscribers that you start with not any that join during that time frame. For effective subscriptions of any kind a low churn rate is fundamental, meaning keeping as many subscribers as possible.