Every domain has an annual fee which is due to the Domain Registrar. Some domain registrars allow for a domain to be renewed for multiple years at a time, while others just allow a single year on each renewal anniversary.
Think of it like your license disk on the windscreen of your car. Except when you fail to renew it, your website and emails no longer work as they are suspended by the Registrar. Imagine if this happened to your car?
Your domain renewal anniversary date is critical, and domain renewal fees need to be paid in full in advance by SA Domains to the applicable Registrar, so remember, this is one invoice you cannot forget to pay, or push out to next month!!