It is interest you are paying, like on a loan, on the unpaid portion of your bill.
E.g. - if your VISA bill was $5,000, and you paid $3,500, you would have an outstanding balance (money you still owe them) on your account. The credit card company would the calculate your finance charge on the outstanding balance, based on their interest rate for that account.
Interest rates differ vary, from person to person, based on your credit history, and their credit policies. They will have a published rate range that you should be able to find on the bill, or on the credit card agreement. They could range from 5% to 30% in today's market.
It is important to pay down your credit cards as quickly as possible, until you can maintain a zero balance, and avoid these finance charges. Never only pay the minimum due. You will never get your card paid off doing that. You may not even be paying the full interest each month. Do all you can to get, and stay debt free.
(Not knowing how much credit experience you have, I provided more information than you probably needed. It's the parent coming out in me...)
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