They are ways that Disney offers discounts. They are specific to the person they sent it to. So if you don't get one, you can't use your neighbors or other family members. Or if you get one, your Husband/Wife couldn't book the trip in their name, it would have to be in your name. No one really knows how to get a PIN code. Consensus seems to be that getting your name on every Disney mailing list possible is the best way to get a PIN.
PIN is just like for your ATM card - Personal Identification Number. Basically a way to make sure only the people they want take advantage of a deal. Be it someone who hasn't visited a certain hotel, or spends a certain amount of money, whatever criteria they decided to use to send out the targeted discounts.