With mobile phones, PUK stands for "personal unlocking key." This code is used to open a SIM card that has been locked after using the wrong personal identification number (PIN) a certain number of times.
Most SIM cards allow only up to three tries before being locked. A locked SIM card then requires a PUK. This usually comes with the SIM card, or the mobile carrier can be contacted to inquire how to procure this code. If the wrong PUK is entered a set number of times, the SIM card is then permanently locked. This can happen if a person continues to enter the PIN when the SIM card is already asking for the PUK.