Open invoices are invoices that still have a balance due. You are able to run a report that will show all open invoices for a specified date range by date, by customer or by aging.
Click on Reports, Revenue, Open Invoices.
By Date lists the open invoices in order by the date they were created.
By Customer lists all open invoices for customer 1, all open invoices for customer 2, and so on.
By Aging lists open invoices totals by how long the invoice has been opened: 0-30 days, 31-60 days, 61-90 days, 90 days or longer.