Backorder is used to get a domain name which is currently with some other owner. It gives a possibility to acquire the domain name if its current owner does not renew it next time.
However, backorder is not as simple as it looks. First you have to understand the process of expiration of a domain name.
Once a domain name registration period comes closer to ending. The domain owner is given reminder to renew it. Due to some reason if the registrant forgets to renew the domain name, the registrar waits for renewal confirmation for some days (40 days for .com). This waiting period is called Grace period. During this period, the DNS, mail and other features of the domain will not work. If registrant renew during the grace period, he will not be charged any extra amount as the late fine.