When you register a domain name, you gain exclusive rights to that domain name for a period of time but not forever. It’s more like a lease than a purchase. In effect, renewing extends your lease on the domain name. Once your lease expires, you no longer have access to that domain name or any services which were tied to it while it was active under your registration period.
In order to avoid downtime, extra fees, or potential loss of your domain name to someone else, it is highly recommended that you renew early or for an extended period of time. Domain names can be renewed (per ICANN rules) for up to 10-years at a time.