A Money Transfer Control Number is a tracking number that only Western Union uses. Any time people use the global payments company to send money, the company assigns their transaction this 10-digit reference code.
Senders can go to Western Union's website and use the MTCN to check the status of their transaction. The MTCN is printed on the sender's receipt. Once senders get this number, it's mandatory that they give it to whomever they're sending the money to. This is critical because the receiver will need to show the number, in addition to proper identification, to the Western Union agent to claim the funds.