You can register any Domain Name for a particular period of time (in years), say 1, 2, 3... years or so. Most registries allow domains to be registered for maximum period of 10 year at once. During this period you gain almost complete autonomy over the domain name you registered. But what once that period of registration is over.
Simple, before the domain name expires, ie. it's registration period comes to end, you need to renew your domain name to continue using it again. Your domain name service provider or registrar informs you in advance about the date of expiry of your domain name. You can renew you domain name at the time of its expiry or you can also renew your domain name well before its expiry date. Most of the professionals and corporates either prefer to register their domain name for multiple years at once, say for 5 years or 10 years at once or do ensure to renew their domain name well before its expiry so that their business is not affected due to expiry of their domain name due to any unexpected circumstances. It makes then retain and maintain their online presence and brand identity without anyone else taking control over it. Because once a domain name expires anyone else can get it registered and gain control over it. Of course once the domain name expires you do get some grace period to get it renewed before completely losing control over it. But the comes at great price. Registries / registrars generally charge huge price for domain renewal at this stage. Their is also possibility of domain getting hijacked by any fraudster who may ask for astronomical price for releasing the domain for you.