Frm Wikipedia
Backorder is a Distribution (business) term that refers to the status of items on a purchase order in the event that some or all of the inventory required to fulfill the order is out of stock. This differs from a forward order where stock is available but delivery is postponed for another reason.

Different computer systems will handle back orders in different ways. Typically received stock is allocated first to back orders. These orders are then available to be fulfilled in the normal way.