Apple doesn't charge anything to your card UNLESS you approve a charge for something you wish to purchase. The Card on your account is very safe and very secure. But a card or funds in your account is a requirement and until you either add a credit card to your account or put money in your account, you won't be able to download any app which include in-app purchases, which is pretty much most free apps.
I have had a credit card tied to my account for over a decade and have never once had any issue with charges I didn't authorize.