An invoice is a generated, visual representation of a particular billing. On the invoice list page you can select the invoice id or select view details from the action menu to display the details for that invoice. You can also write off the invoice from the list page (as well as the detail page).
The Invoice page displays invoices by Invoice ID, Amount and Currency, Status, Invoice Date, Due Date, and Balance. You can search for invoices by any of these key words in addition to customer and subscription identifiers, click the Show Filters button to refine your search.