A PIN is a 16 digit numerical code that Disney sends out at random to people either via email or as a postcard in regular mail. The PIN must be called in to Disney to take advantage of the special rate that is offered. Sometimes they are for percentages off room rates, other times it is for
free dining, or kids dining & tickets for free.
No one knows for sure exactly HOW to get these codes, but one of the easiest ways to make sure you are on Disney's "list" is to sign up for the informational DVD and at the website for emails (which come from Disney Destinations).
These codes are not transferable, and are not offered to the general public (like the current Free Dining that is being advertised on TV), so only a select "few" receive them. Disney also only has a certain number of rooms for each resort for all the different offers that are sent, and most have restrictive dates for the offer (which is why you see some people here talk about it not being during their travel time).