But as others have said, anything you don't pay off incurs interest charges the following month.
And if you don't pay it all off the following month you are getting interest on that interest.
Then the next month interest on that interest on that interest and so it goes on.
And the interest rates are high.
Credit cards are great way to buy things, but an easy way to run up huge debts if you don't pay off the total amount each month.