Under Add Funds you can load money onto the account before you owe it - a few preset amounts, or a custom one.
Credit is applied to invoices as they are issued, which is useful in two situations: renewals you never want to bounce (the invoice pays itself from credit the moment it is issued), and accounting flows where one larger payment is easier to process than many small ones.
Your current balance shows on the same page and on invoices as they consume it.