The difference is very subtle. Food implies something that you eat. Foodstuff is, as the dictionary says, anything that can be used as food. So when the FDA says "foodstuff," they are including raw ingredients like flour and baking soda, and spices like garlic and cumin. These things are questionably "food," since you wouldn't eat any of those things alone. However, people don't use the word "foodstuff" very much, and will often just say "food."
A serving is the rough amount of a food that you would eat in one sitting. For example, one serving might be 15 potato chips, one potato, one bottle of juice, etc. It is a very approximate measurement. All packaged food in the United States defines on the label what "one serving" is, and then lists nutrition information for that one serving.