You get a certain number of credits for each night you stay.
(1) Counter service meal
(1) Table service meal
(1) snack
We did free dining last September, and I kept track of how much we would have spent on food. We got well over $1200 of food for free. Is it worth it? Everyone will have their own opinion, it just depends on what you would regularly do. If you would regularly just eat at lots of counter service, probably not. If you want to do lots of table service, then I would say definitely!
If you do it, BE SURE to make your reservations early, they book up fast during free dining!