back order mens the required quntity for the sales order is 
not availble in the inventory thats why it si not shipped 
and is back ordered.

There are few reasons that a scheduled order could be 
backordered during pick release process.

1. In-sufficient onhand
2. Order might be put as 'Pick release hold'
3. If it's a lot controlled item being involved then some 
of the factors will be checking at picking rule setup (like 
Lot- expiration date, etc) and tured to 'Backordered' 
incase the criteria does not meet.

back order means.
1) sales order is raised by the customer for a item & on 
hand QTY of that item  is zero.
2) so to full fill the order a purchase order(requisition 
import) raise to the supplier .
3)item is recived in the inventory.
4)customer order is full filed .