What is a PUK code?

A PUK code is a security feature on your sim card that, where you can choose to have your sim request a PIN code when your phone is switched on. If this code is entered incorrectly three times, your sim card will lock itself, and this can only be unlocked using your PUK code. As your sim card is controlled by your network, it is your network who have this code- you will need to contact them and request this.