Personal Identification Number.
In the context of wireless devices, the PIN is a like a password that prevents other people from using your service. It is numeric code used to unlock the phone and/or complete a call or data transmission. It must be entered each time you use the phone, unless you disable it.
(Note that this is different from the type of "unlocking" that will let you use your phone with a different carrier.)
In GSM phones, the PIN is generally associated with the SIM card, not the phone.
GSM SIM cards will lock up if an incorrect PIN is entered three times in a row. If this happens, a different, longer code called a PUK code is required to unlock the phone.
See: PUK code