When you pay with your debit card the money is directly withdrawn from your bank account (to which the card is connected).
When you pay with a credit card you postpone the payment; you actually borrow money from the bank/creditor. Then of course you have to pay the bank/creditor back. It will be more expensive since there will be an interest rate on the borrowed money. Some cards have very high interest rates. Read our articles here in MyWallet.pw to help you find the best credit cards for your needs.