Churn rate is a measure of customer attrition, or turnover, within a company. It's used to calculate the percentage of customers who cancel or do not renew their subscriptions to a service within a given period. Churn rate is important because it can indicate whether a company is losing or gaining customers. A high churn rate may be indicative of problems with the product or service, while a low churn rate may suggest that customers are satisfied and loyal.