This is an entry in our Disneyland 101 Glossary: Disneyland Resort definitions in under 101 words, from Mouse Brief.
Dining reservations (referred to by some Disney fans as ADRs or Advance Dining Reservations) can be made for table service restaurants and dining packages at Disneyland Resort up to 60 days in advance. Unlike Walt Disney World in Florida, this reservation window is not affected by whether you have hotel reservations. You can make reservations online, through the Disneyland app or over the phone. There is a credit card guarantee with a fee for no-shows and late cancellations. Some restaurants in the Downtown Disney District are not part of the Disney reservation system and may handle their own reservations.