You could search the forums and you'll see many threads explaining it. It's also in the FAQ. Basically, you need to pay the fees associated with the transactions. These are fees charged by credit card companies and PayPal. (Itch.io doesn't charge anything, but they do take whatever you give them off the top - not after the fees).
When you request a pay out, there are again fees involved to process the payment charged by PayPal etc.