When you pay for domain name you're not buying it, but renting it for a certain period of time. That means that it has expiration date set. If you want to use that domain name after expiration date, you have to renew it's registration for at least one year.
If you have forgotten to extend domains' registration before it has expired, it enters into the REDEMPTIONPERIOD for 30-60 days after which will be deleted from the registry.
From that moment you cannot renew it unless you contact us. There's a $300 fee to be payed so you do not loose your domain name.