A PreBill invoice is a draft invoice. It is editable, and you can click right on the time and expense entries in the PreBill Invoice. The original Time Entry and Expense Entry windows will pop-up, where you can make any edits. When you click "Save" the PreBill Invoice automatically updates with your changes.
All invoices pass through the PreBill stage before you finalize and send them to Clients for payment. In order to send an Invoice to a client, you must click the "Finalize" button on the Invoice. Finalized invoices are locked, and can be tracked in the Accounting Tab of Time and Billing. PreBill Invoices do not show up in the Accounting Tab.
You can search for and see all PreBill and Finalized Invoices on the "Invoicing" Tab, "Invoice List" Sub Tab.
See an example here: