The Invoices section is where you can find all of the invoices generated across all of the projects which you're assigned to. By default, if you're the admin of your OS, you'll be assigned to all projects which come into your OS.
From here you can also access the Invoice Settings and create a new Invoice, which you can learn how to do from looking at the article on Projects. The only difference is that, since you're not starting from within a project, generating an invoice from this page would require you to first select which project the invoice will be assigned to.
This is key: invoices cannot be without projects. If you find that you'd like to create an invoice, have your customer make a project first, or make one for them. This will also allow the invoice, and all work done as a result of it, to be covered under the Client-Provider Service Agreement.
That's all you need to know about the Invoices section! If you have any questions, drop us a line at firstname.lastname@example.org and we'll be happy to help!