Domain names must be registered and renewed every year. You can pay for a domain name's registration several years in advance, and even register a domain name "forever" (Epik.com currently offers a 'forever registration' option). Every registered domain name has an expiration date, the date when that registration expires.
If not renewed by the expiration date, a domain name goes through a "drop" process that ultimately leads to it being available for anyone to register again. DNProtect recommends that you register your domain name for at least 5 years in advance, which is our recommendation, which leads to less risk of losing the domain name.