It is advisable to renew your domain name before the expiry date if you intend to keep it to avoid having your account deleted and risk losing the domain name to a third party or paying an additional registrar cost to renew it.
If the domain name is not renewed then it will be suspended by the registry. In the case of .gTLD domain names after this period it goes into a Grace Redemption Period, during this period there would be an additional registrar cost if you were to renew the domain name. After this period the domain name can not be renewed or registered for a further 5 days and then would be available to registered by anyone.