A Private Unblocking Key (PUK) code is a safety measure used to guard SIM playing cards from unauthorized entry. If a person enters an incorrect PIN (Private Identification Quantity) thrice in a row, the SIM card will likely be locked and the PUK code will likely be required to unlock it.
The PUK code is often offered by the cellular community operator when the SIM card is bought. You will need to maintain the PUK code protected and separate from the PIN code, as getting into an incorrect PUK code 10 instances in a row will completely block the SIM card.