A "MAC" code stands for "Migration Authority Code" and is issued by broadband providers when you want to change broadband providers.

The "MAC" is used between broadband providers to help migrate customers from one broadband provider to another. Mnay of the broadband providers signed up to a code of practice to allow the MAC code to help the end user.

It is similar to the PUK code you have to request of you are changing mobile telephone provider and want to keep the same telephone number.