DDE, the Disney Dining Experience, is a card that Annual Passholders can get. It gives them 20% off table service meals at certain restaurants, including alcoholic beverages.
The cost per year is $60 for annual passholders and $85 for Florida residents. Florida residents can renew for $75. There's a $25 charge for a second card.
It is accepted at most WDW table service restaurants, bars/lounges in parks and resorts, and a few counter services. There are eight blackout dates, mostly major holidays. As noted, the discount is 20% off the full bill, including alcoholic beverages. An automatic gratuity of 18% is added at all locations except counter service.
Many folks researching a WDW trip confuse the Disney Dining Experience with the Disney Dining Plan. These are two entirely separate programs.