Merchant Category Codes commonly known as an "MCC" is a four digit number that classifies the type of goods/services a specific business offers.
Each type of business has a different MCC code assigned which allows the Card Issuer to monitor what cards are being used for. The card issuer can block an MCC if it is deemed too high risk or the MCC has not been enabled for you to use.
To learn more about MCCs click here.