Interchange is the fee paid to the card brands for the cost of accepting credit cards. Every time you accept a credit card, you pay a fee to a credit card processor. That fee has three parts: interchange, assessments, and processor markup.
Interchange is the largest part, and it goes to the banks that issue cards to customers. It is also non-negotiable; Visa and Mastercard set the interchange rates for accepting their cards. Additionally, they publish those fee tables, which include dozens of interchange categories.