What Is on the Shelf
Food adds levels, and each level is one point to put on CP or HP. Every food takes a different number to make a level — cheap food takes more of it. The shelf says how many, and your pantry says how close you are.
Your Pantry
Food is bought once and eaten whenever you like. A card eats enough for a whole level at a time, so anything left over waits here until you have enough for the next one — nothing is ever wasted.