What is a PUK Number?

If it is asking for a PUK (Personal Unlock Key) that means that your SIM has a passcode lock, and you forgot it. You can get the PUK by calling your cellular provider or by logging in to their customer portal.

Be careful not to enter the wrong PUK number too many times. If you do, it will destroy the SIM card and you will need to have it replaced by the carrier. As Lawrence Finch said, your carrier is the only one that will be able to help with the PUK number.