Your credit card balance shows the amount that you owe to the credit card company. If you have an outstanding balance, your credit card balance will not have a sign.
Charges on credit cards increase your balance and show up as a positive number on a statement. Payments and other credits, on the other hand, show as negative numbers.
This is the reverse when using the /transactions endpoint for debit accounts where charges decrease your balance and show up as negative. Payments and other credits show as positive.
Check the example below from AMEX:
