Customer lifetime is the entire period between first and last purchase of one client. It is how long they stay a customer.
Depending on the products, customer lifetime can be very long or very short. An online grocery store can serve people throughout their entire life, while a bridal dress store can hardly expect more than one order from a person.
Longer customer lifetime means better retention rate and higher customer lifetime value so it’s worth trying to prolong it if your products are suitable for that.