The order status awaiting invoice indicates that the order has no invoice yet assigned by either yourself or the supplier.
The invoice tab will indicate who is responsible for submitting the invoice:
Paper: You the customer as these will be orders placed with your Ad hoc or self-managed suppliers.
Portal or EDI: Your supplier is responsible for submitting a copy of their invoice to the order.
More info
What does it mean? The invoice has not yet been submitted.
How did it get there? A purchaser user confirmed the delivery before the invoice was submitted.
What is the next step? If set to paper, purchaser must submit the invoice. If set to portal/EDI wait for the supplier to submit or contact the supplier to chase completion.