The total revenue (or profit) a customer generates over the entire time they're a customer.
LTV is how much a customer is worth to you, forever. Simple calculation: average monthly revenue × average customer lifespan in months (or: revenue / churn rate). If customers pay $100/mo and stick around 24 months on average, LTV = $2,400. Sometimes calculated on revenue, sometimes on gross profit (margin-adjusted LTV is more honest).
LTV is the ceiling on what you can spend to get a customer. Spend more than LTV and you lose money on every sale. High-LTV businesses can afford aggressive acquisition; low-LTV businesses can't. This is the core math of customer-based businesses.