Interchange is the cost the card networks (Visa, Mastercard, Discover, American Express) charge to move money between the buyer's bank and your merchant account. It's the wholesale rate — every processor pays the exact same interchange. What varies between processors is the markup they add on top.
Interchange rates vary by card type (debit is cheaper than credit, basic credit cheaper than rewards cards, rewards cheaper than corporate), by entry method (chip-inserted is cheaper than keyed-in), and by industry MCC (gas stations get special low rates, charities get reduced rates). A typical retail interchange rate is 1.51% + 10¢ for a Visa rewards credit card; a card-not-present transaction on the same card can be 1.80% + 10¢.
Because interchange is the wholesale floor, the only rate any merchant can ever 'beat' is their processor's markup. This is why interchange-plus pricing — where the processor explicitly shows interchange separately from their margin — is the cleanest model.