The PUK code stands for "Personal Unblocking Key". The PUK code can be obtained by calling your carrier's Technical Support Department and verifying account details. Also, they might have you verify the Serial Number on your SIM card to make sure you have the right PUK code.
Heed warning. At first, if you enter the PUK code incorrectly 3 times, it will prompt you differently. If you continue to enter it 10 times wrong after, your SIM will be completely locked and will be unusable. It's best to contact your carrier as soon as the device requires the code!