It is a code for some deal that is sort of like their special offers section. It could be a percent off, a special package (like gift card with purchase, free dining, etc.), or Kids Go Free with Adult Purchase, things like like.
The difference between a PIN and the Special Offers section is that while the Special Offers section is open to anyone to use, a PIN is a number that is only assigned to you - select people get them, not all potential visitors.
No one knows how PINs are assigned to people, there is no rhyme or reason. But it happens, and when it does, it's pretty cool, if you can use it.
Was that a good explanation? I'm too busy checking my email every sentence to know if I adequately explained.
Must. Walk. Away. From. Computer.