What does it mean?
A metric that measures how many customers you retain and at what value. To calculate churn rate, take the number of customers you lost during a certain time frame, and divide that by the total number of customers you had at the very beginning of that time frame. (Don't include any new sales from that time frame.)
For example, if a company had 500 customers at the beginning of October and only 450 customers at the end of October (discounting any customers that were closed in October), their customer churn rate would be: (500-450)/500 = 50/500 = 10%.
Churn rate is a significant metric primarily for recurring revenue companies. Regardless of your monthly revenue, if your average customer does not stick around long enough for you to at least break even on your customer acquisition costs, you're in trouble.