"Backorders" refer to items that a purchased, while it is out of stock. Back ordered items usually take slightly longer to ship; as they are shipped as soon as the item is back in stock.
Dispatch times for back-orders vary, each depends on when the item is re-stocked. The time (in most circumstances) only differs by a matter of days. In extreme circumstances .i.e. high demand, this difference could be a matter of weeks but nothing longer.
If you order a back-ordered item, you will be notified within 1-2 working days of an estimated shipping date.