A PIN, or personal identification number, is a four-digit number that must be entered into the keypad to unlock the phone for use when the security feature on the phone is activated. The PIN is built into the phone by the manufacturer.
Setting up a cell phone to be unlocked with a PIN ensures that, if the phone gets lost or stolen, it is no longer usable, and that no one else has access to the personal data stored on the phone. Having unauthorized possession of an unlocked cell phone is likely to let the thief see personal texts, contacts and notes. The person also has easy access to any email accounts, social networking accounts, online shopping accounts and even bank accounts that the cell phone owner may not have logged out of, making it possible for him to change their passwords.
Locking a cell phone with a PIN also stops the inadvertent dialing of a pre-programmed number when the phone is bumped around in someone's pocket or bag. The phone's SIM card can also be locked with a PIN. Some phones are set to erase all information if an incorrect PIN is entered into the phone too many times.