Customer churn is the percentage of customers that stopped using your company's product or service during a certain time frame. You can calculate churn rate by dividing the number of customers you lost during that time period -- say a quarter -- by the number of customers you had at the beginning of that time period.
For example, if you start your quarter with 400 customers and end with 380, your churn rate is 5% because you lost 5% of your customers.
Obviously, your company should aim for a churn rate that is as close to 0% as possible. In order to do this, your company has to be on top of its churn rate at all times and treat it as a top priority.