Food and drinks are considered nourishment and have restorative properties. They are available from a variety of locations including. but not limited to, vendors. For example killing a chicken will net you one chicken leg. The easiest way to obtain nourishment is through looting during exploration. If it is on a table, a pot, crate, basket etc you can pick it up and add it to your inventory. Each type of food/drink item occupies one slot in your inventory, but items of the same type stack. You will not get any kind of negative rating for looting food in front of NPCs. The items looted will not be replenished. Of course you can always purchase food and drinks from a vendor, the cost of the item will depend on how they feel about you (many merchants can be mindread to lower their prices). Selling food and drinks to vendors is not a good way to make money, you usually only get one gold coin for the item regardless of how the vendor feels about you.
Foods are consumable items which heal hit points (HP). They are most useful at the early of the game in lieu of using a more costly potion.
Drinks typically boost the player's mana. Like foods, drinks are useful early in the game instead of using a potion.