Mostly after a bank transfer order, but this can also happen if e.g. does not directly validate the transaction, the order is at a pending state. This means the order is not fully paid. All pending and/or not processed orders will be shown at the "Pending-Orders" list.
To set an order to paid, click at the "Payment" button and enter the paid amount at the popup dialog. The missing amount is set by default, but you can set any amount, e.g. if the customer sends the money in rates.
If the order is fully paid, the button is hidden and its time to process the order. This will mark the order as processed and an new period will best stored, depending the product and subscription type / time ordered.