Retention involves engaging and re-engaging existing customers to maintain their loyalty and service. This, in an ideal sense, is achieved by providing a product or service they simply cannot get elsewhere, or by having a brand that stands out among its peers. In most markets, however, retention is a competitive strategy, utilizing reward programs, exclusive access, and/or other perks and offers to keep customers coming back. 

Retention, like acquisition, provides revenue for your business through repeat buys. More than that, though, high retention rates demonstrate loyalty and a willingness to choose your company over another. These repeat customers are more likely to provide positive reviews, to tell others about your brand, and to influence others to purchase your products or services.

References

https://en.wikipedia.org/wiki/Customer_retention

https://www.shopify.com/blog/customer-retention-strategies

https://www.superoffice.com/blog/customer-retention-tips-with-crm-software/