Barbacoa is meat (beef, goat or lamb) which is cooked in an underground pit. The chile-marinated meat cooks slowly over a period of 6 to 8 hours. The broth is collected in a pot at the bottom of the pit and is used to make consomme which is served as an appetizer. The meat is served with tortillas so each diner can make their own tacos, and in the photo above, with beans and "masita" (cracked corn which is baked in the oven with the barbacoa).
Barbacoa is a special occasion meal customarily served on Sundays, and also at large family fiestas such as weddings, quinceañeras and baptisms. If you're not invited to a private party you can sample some pit-cooked barbacoa at La Capilla restaurant in Zaachila or at any of many roadside stands or market stalls which sell barbacoa on Sundays.
Dedicated carnivores should also not miss having a meal in the pasillo de carnes asadas (grilled meats hall) in the 20 de noviembre market.