Annual Domain Renewal Is Mandatory
Your domain should be renewed every year. The renewal amount may differ depending on the Domain registrar. If you dont renew your domain within the expiry date,a specified period will be given to you to renew it after the expiry date(for example- 15 days for .in , 30 days for .com, etc) but you may have to spend 10 times the actual renewal amount to get back your domain in this period.
If you still dont renew it, after a couple of months, you will lose your ownership over it and the domain will get released in online market for others to buy. Once another person owns your domain, you will no longer have any authority over it and all the energy, time and money spent by you on optimization and promotion of your domain will go invain.
Eventhough if you want to buy that domain again , you have to wait till that domain gets released into market once again or you may never get it back. So think twice before you let go of your precious domains.