A credit on your account are additional funds that have been sent to us, other than just paying for your server. You can add more credits on your account by logging into the client side panel on tserverhq.com and going to your account, click the "Add Funds" button and add how much you want. Our automated billing system will create an invoice for you. When you pay that invoice you will then have more credit on your account. The system will then use that money to pay for the servers until the credit runs out.