CAC is the Customer Acquisition Cost and it is what you spend to get a customer.
Lifetime Value is what that customer is worth over time.
The most important ratio in your business is making sure the second number is significantly larger than the first.
If it costs you $500 to acquire a customer who spends $5,000 with you over their lifetime you have a healthy business. If it costs you $500 to acquire a customer who only spends $400 you are paying to lose money.
Know both numbers. Build the gap between them as wide as possible. That gap is your business.