An Account Credit is a prepaid balance deposited into your Heroic Services client account that can be used to pay for future invoices automatically.
When an unpaid invoice was already present prior to having credit added to your account, credit needs to be manually applied to pay for this invoice.
You can add credit to an invoice manually in the My Invoices page within the client area.
You're done! The credit will instantly be applied to the invoice. If you had sufficient balance to pay the amount due in full, this will update the status of the invoice to Paid.