The term backorder means "to bring something back".A backorder-service or a drop-catcher is a provider who snaps RGP-domains (also called expired domains or dropped-domains). These so-called RGP-domains will be caught and registered after the grace period (at nicsell in customer orders) ends.
In order to offer such a service, you have to be a registrar at DENIC. Nicsell is a backorder-service that can register deleted domains, if the auctions are won by the customer.
Once you are logged in at nicsell, you can see the comlplete domainlist and see if the domain you are looking for is in there too.
Please note that the domains appear in the domainlist only 2 weeks before the expiry of the grace period. From this moment you can bid directly an auction our backorder-service.