Vittles restore health when consumed, and they are identical in use as Medicine and will regenerate around 80% of max health. As with all other Consumables, they can only be used in single player.
Vittles can be made from almost any type of animal. Birds (chickens included) are the only animals that don't provide Vittles, as the player only collects their feathers and does not skin them for their meat. Although one can procure Vittles from small animals as well as large animals, occasionally smaller animals may not yield Vittles while larger animals always do.
What's more, obtaining Vittles does not prevent one from also harvesting the animal's pelt, flesh, or other collectible goods. These items are still added to the player's inventory as usual and are, of course, able to be sold at stores. However, Vittles cannot be sold in stores, as they are a consumable item.