A foodstuff is food when you are talking about it as a thing that is grown, made, stored, or sold — not as dinner on a plate. You meet it in business, farming, trade, and official writing: grain, oil, sugar, tinned fish. It often appears in the plural, foodstuffs. You would not normally tell a friend you bought foodstuffs at the shop; you bought food, or groceries. The word is dry and useful. It lets a report talk about ‘basic foodstuffs’ without naming every item. Stuff here means material or goods, not ‘rubbish’. Though actually, in ordinary speech stuff already covers this in a looser way — ‘food and stuff’ — but foodstuff is the tidy, official member of that family.