What is a PUK code
PUK is the code required to unlock a GSM SIM card that has disabled itself after an incorrect PIN code was entered 3-5 times in a row. See: PIN. If an incorrect PUKcode is entered a certain number of times (typically 5-10), the SIM card will permanently block (disable) itself.
If in the settings you chose to lock the SIM(specifically) so that someone cannot take it to another phone and gain access to your service and then used the wrong password multiple times you would be locked out of your phone. You will need to contact support to get a code to unlock it again.