So here's how credit cards work...

YNAB makes a budget catagory for the credit card account. If your cc balance is 0$, the budget catagory would show 0$. If you came into Ynab with a balance of, say, 100$ it would show -100$ and you would owe money and would have to budget 100$ towards it to pay off the credit card debt.

Now, once a CC account is in YNAB, when you make a new CC transaction YNAB is going to ask which catagory the transaction falls under (just like any other transaction) and you have to tell it! When you do, it moves money from the catagory you specified (say Grocery) and moves it over to the CC balance catagory so you can pay off the CC transaction (Ynab does not want you to carry debt, you spend only what you have).

So if your CC balance was 0$ and you buy 20$ in Groceries your Grocery catagory drops from 100$ to 80$ and your CC balance goes from 0$ to 20$ to pay off the 20$ you just put on the card.

If you had a balance on the card of -100$ then Ynab still moves the 20$ so your Card Balance catagory would look like -80$ but as soon as the CC transaction clears it puts the 20$ on the card so you still owe the original 100$ on the card and have to move money on your own from some catagory to fully cover the debt.

But once a card is zeroed, you do not have to worry about anything since Ynab automatically moves money to cover new credit card transactions.