A backorder is the intention to purchase from a customer before the domain name is released into the public domain. It amounts to the reservation of a domain before "snap". All you have to do is backorder it so that it can be assigned to you after the snap by our service. This is not a purchase, but a reservation.
This concerns domain names that have not been renewed by their current owner and are in a period of redemption, meaning they are pending, before being released and put back on the market. When a domain name is in redemption, it is deactivated and no longer works, but it still belongs to its previous owner. The latter can still request a restoration, by contacting his registrar.
To know everything about the price of pre-orders, check out the following article: "What are the steps involved in the release of a domain name?".