When you want to get a domain name that is currently registered and about to expire, you have two options. You can wait until it goes through the expiry process (which can take up to 90 days from the actual expiration date) and then register it yourself at any Registrar when it becomes available to the public. Or, you can buy expired domain names by placing a backorder with SnapNames.
The domain expiry process can be confusing, and there is no way to tell if the current owner will renew the domain name or not. If you have time to wait it out and don't need to make an offer to buy the domain name from the current owner, placing a backorder is the easiest way to try to secure the name as it expires.
When you backorder an expired domain name, you are telling SnapNames to try to acquire the name when the current owner does not renew it. This saves you from having to check to see if it has become available, and reduces any chance of someone else registering it before you.