Outstanding means the amount to which you are exposed as a business, which in layman's terms means the amount which you are owed for that Order has not been authorized by the Customer.
If payment for an Order is authorized, it means you are able to collect payment for that Order by Charging the Card, hence the business is less exposed. In this circumstance the Order will show an Outstanding amount of $0.00.
If the payment is not authorized, the Order will show an Outstanding amount. In this circumstance you should do the following:
1. Search for the Order
2. Click 'View/Edit'
3. Click 'Reconfirm'
4. Send a payment request email to your Customer
The Customer will then enter their Card details via a link in the email they have received.