There is a whole section on the dining plan. Various websites also have wonderful helpful information. I'll give you some basic information though. For each night of your stay at your disney resort (nights, not days), you receive a table service credit (appetizer, entree, drink and dessert), a counter service (entree, drink, dessert or combo meal) and snack (usually under $4) per person. Note that each person gets an appetizer for table service (not 1 for the whole group). There is a ton of food. Not all restaurants participate, but many do. (I think over 100). You can exchange a table service for 1 character meal, or you can exchange 2 table service for a signature dining restaurant (very expensive restaurants that are fancy), or a dinner show.
Pros: Meals are paid in advance, tax, tip included, saves a lot of money.
Cons: Not worth it for people who don't eat a lot or who usually just order an entree
The dining plan can save you hundreads if you go to more expensive restaurants and order the more expensive entrees. Also, the meals are pooled into 1 account so anyone in your group can have a meal any time any day. You can use all your snacks in 1 day or use 2 counter service in 1 day and then 2 table servie the next day. It's up to you so there is a lot of flexibility. I definetely love the Dining plan!