Customer Lifetime Value (LTV) is an estimate of the average gross revenue that a customer will generate before they churn (cancel).

Why should I care about LTV? Primarily to apply a limit to your Customer Acquisition Cost (CAC) - if you're spending more on acquisition than you anticipate to earn from the customer in revenue, you're going to be in a bad place very soon (unless you have a bottomless pot of money to spend on acquisition).