Marking an order as complete is the last step in the order processing cycle.
Only 'Confirmed' & 'In-Process' orders can be marked as complete.
When you mark an order as complete, a notification is sent to your buyer & to all your linked Sales Reps. If custom order fields are set, then the dialog to complete the order, displays those fields.
Once the order has been marked as 'Complete', no further editing can be done on that orders.