Credit card debt is any amount that you owe on your credit that is past the due date. This is also known as carrying a balance on your card.
Each time you leave a balance on your card, you are subject to interest payments (and late fees if you don’t make at least the minimum balance). This causes the amount you owe to drastically increase each month due to the traditionally high APRs on credit cards. It’s easy to see how this can get out of hand quickly.