Customer Life Time Value. The total amount of revenue that a single customer will generate in all it's lifetime using the app. This is a very important metric for subscription based businesses (like saas).
For instance if a customer pays 30$ monthly, and stays with you for 10 months, then the CLTV for that customer is 10 x 30$ = 300$. It is usually calculated by divind the ARPU by the churn rate. In saas this is usally around three times the CAC value.