By enabling Delete, you will be able to delete any invoices you no longer want via the Quick View of that invoice.
To delete an invoice, enable this option, then go to the Manage Invoice page and select the Quick View for the invoice you wish to delete. In the Quick View screen there will now be a delete option in the actions menu. Click this button and follow the prompts - Your invoice will now be deleted
Note: Currently only invoices can be deleted, but in the near future this will be extended to all the other sections (ie. billers, customers, etc..)