What does backorder mean?

When you have gotten an order which includes one or several products which you don't have in stock, these orders will become a backorder. This could be due to your stock balance is set as an Oversellable stock on the product(s). This orders will end up in the Pending Unstocked section within Warehouse (WMS). To handle and deliver these orders you need to fill your stock with the missing product(s), once the new stock is delivered in the system you may handle the order as normal.