A PUK or 'Personal Unlocking Key' is a security feature on most mobile devices that protects your SIM card data. The PUK code is unique to your SIM card and is required when a SIM Card PIN code has been entered incorrectly multiple times.
What is the unlock code for?
These are usually referred to as "Network" (NCK) or "Service Provider" (SPCK) locks and can be removed using the corresponding unlock codes, which are unique to each phone depending on its IMEI. Typically, a locked handset will display a message if a restricted SIM is used, requesting the unlock code.
What is a Network locked SIM card?
A SIM lock, simlock, network lock, carrier lock or (master) subsidy lock is a technical restriction built into GSM and CDMA mobile phones by mobile phone manufacturers for use by service providers to restrict the use of these phones to specific countries and/or networks.