It is a hex set that identifies your specific phone, it is used in conjunction with the blackberry messenger program. Which is basicly an IM for bb users only.
It actually goes beyond that. It serves like a VIN for a vehicle. Each BlackBerry device has it's one PIN and it does not change, even if ownership does. Yes, BB Messenger uses it for Instant Messaging, but other programs can use it for identifying transactions made by each BlackBerry user.
