Dictionary.net

an accumulation of jobs not done or materials not processed that are yet to be dealt with; "a large backlog of orders"

an accumulation of jobs not done or materials not processed that are yet to be dealt with (especially unfilled customer orders for products or services)

The value of orders that are not yet filled or the undone jobs on any day. This shows that there is area for growth in teh company. AKA open order.