When you purchase a domain on Sav, you do not own it outright. The registration is like a lease and you have exclusive usage rights for the duration of that lease. Registrations typically last between 1 and 10 years and need to be renewed when that registration expires to remain active.
You may add years to your domain registration by renewing at any time, even if the domain is not set to expire soon, but you cannot renew for more than 10 years at any time. For example: if your domain registration is set to expire in 5 years, you may add up to 5 more years to your registration.