Upon registering a domain with a registrar, an expiration date is set for a period of time in the future (at least 1 year from creation or renewal). For all active/in-use domains it is critical to prevent your domain from expiring. Once the expiration occurs all services are shut off causing visitors/customers to your site to be un-able to reach your domain. There additionally is no guarantee your registrar will provide a grace or redemption period.
What happens after a domain expires?
This process varies by provider. Typically there is a grace period and then a redemption period where the owner can renew the domain. Once those periods pass, the domain name is then typically set for auction or deletion and returned to the registry. Beware, grace and redemption periods are not neceassarily guaranteed/can change or be eliminated at the registrar's discretion.