I am trying to make a purchase but it keeps denying. Can you tell me why?

Not sure why you think I'd know anything about your account and your card. It could be being declined for a number of reasons e.g of it's a debit card or a prepaid card, or if not registered to exactly the same name and address as on the iTunes account, or not issued by a bank in the country where you are (and which is therefore the country on the account), or the card issuer might be declining it.

What dInes ‘keeps denying’ mean, what happens when you try to buy something ? If you are getting an error message what does it say ?

It is very frustrating that my payment method keeps declined. What's wrong?