Customer lifespan measures the average length of usage for active users of your product. So, if a new user joins in Jan 2015 and stops using in Jan 2019 then their lifespan has been 4 years. When looked at as an average across all your users you get a sense for the stickiness of your product.
Sticky products are great. Things people love to use, things people can’t get by without. Users value sticky products and are less reluctant to spend money on them.
Conversely, products that are hard to use or do not provide sufficient value are likely to suffer from transient users and a low customer lifespan. This means that all those costs of aquisition are wasted and you may not recoup them due to a low customer lifetime value. Bad.