CAC is customer acquisition cost and LTV is the lifetime value per customer. The CAC:LTV ratio compares the cost of acquiring a customer versus the value that the customer will bring over their lifetime.
LTV/CAC ratio must ideally be 3:1. This is to say that the company can make 3x what it would spend on customer acquisition. If LTV/CAC is under 3, it's an indicator that the company may be spending far too much on a customer that does not fetch value to the business.