I am not really sure how you get them, they just come in the mail periodically. They are cards that provide a discount for the resorts.
Last year I got one that gave rates for all three levels of resorts, this year I got one that gave a rate for the moderate resorts only.
They have a pin # that corresponds to your name and address so they can not be given to someone else to use if you can not take advantage of the rate.
There are also e-mails that give you the same deals. The people that have gotten these indicate that when they sign up on the disney website they check the boxes that ask if you want to hear from Disney regarding promotions and specials