When you register a domain it's for a fixed period of time. At the end of that period you need to renew it or the registrar will release it, enabling someone else to buy it. You really don't want that to happen - it's like going to work one morning and coming home to find that not only has someone else moved in to your house whilst you were out, but that they also own it!
The company you used to register the domain should let you know well in advance of its expiry date, giving you plenty of opportunity to renew it before it expires, but what if you don't get the notification? What if it gets swallowed by a spam filter and you never see it? Or what if the person who registered the domain has left the organisation?
SiteSentry will check your domain registration every day and start warning you 30 days before the domain registration expires, so you never risk losing a domain by accident.