Lifetime value (LTV) is the profit that you generate over the life of your customer's relationship with your business.
Intuitively, if the amount of money that you make over the customer lifetime is more than it costs to get the customer, then you've got a pretty good business model.
There are several ways to calculate LTV. This is the simple way which is sufficient for most early stage companies.
LTV is an important component of your unit economics.