Can someone explain what a MAC number is and what do I do with it when I change servers

Do you mean changing internet service providers?

If so a MAC code is basically a number that identifies your line and the service that is on it. When you pass it on to a different ISP it allows them to take over the existing service so you can move between ISPs without having to disconnect the service and pay for it to be reconnected (ie it would allow you to move from BT, say, to Zen or Tiscali or whoever).

Hope that helps,

Isn't that a PAC code?

No, as far as I know a PAC code is for moving your mobile phone number between mobile providers.