An incomplete order happens when a shopper reached the payment page, but did not complete the payment.
Incomplete orders can be the result of a failed payment attempt (e.g. credit card was declined), because the shopper decided not to complete checkout, or they never attempted payment. This does not include orders where the shopper abandoned the order before making it to the payment page.
Incomplete orders have an order status of "Pending", and can be found under the More tab on the View Orders screen.