Each time an order from your shop is validated, an invoice is sent out to the customer. The "Invoices" page under the "Billing" menu enables you to show all invoices had made in your shop, create new invoice to customer or filter invoices by who create them.

Filters

creating new invoice

Click on "New Invoice" button to add new invoice manually from this page, there's a modal will appears with many fields, let's explain this modal: