Interchange fees are transaction fees that are charged by Visa, Mastercard, American Express or whoever the underlying card network is to your payments provider, and are charged when accepting card payments from customers, whether it's online or in-store payments.
The total fee that you pay your card payments provider includes these charges as part of the total charge.
The interchange fees are calculated based on a multitude of factors, such as the type of card the customer uses, the country of card issuance, and whether the transaction happens online or in person. These calculations primarily take into account the level of risk involved by the card network work providers in providing the card services. This is why international card not present transactions have higher fees attached than in person local cards.
Mastercard and Visa publish their rates at six month intervals and there are a bewildering range of rates, which is why your card payments provider will often bundle the rates and bear some of the risk with you.