A spoonful is how much a spoon holds — the amount you can carry in one spoon without it falling off. 'Add a spoonful of sugar' means add as much sugar as fits in a spoon. It is a way of measuring when you don't need to be exact. In cooking and in medicine, English usually says which spoon it means: a teaspoonful (small), a tablespoonful (big). On its own, 'a spoonful' is a friendly, rough amount — nobody is checking. Two things about how it behaves. First, the plural is 'spoonfuls' — the -s goes on the END, not in the middle. Some careful older writers use 'spoonsful', and it is not wrong, but almost nobody says it now, and your students should write 'spoonfuls'. Second, there is a real difference between 'a spoonful of soup' (one spoon's worth, an amount) and 'a spoon full of soup' (a particular spoon, and it happens to be full). Same sounds, different meaning, and the space is what tells them apart. People also use it loosely for a small amount of anything you might eat or take — 'just a spoonful, I'm not hungry'. And in cooking instructions you will meet 'a heaped spoonful' (piled up) and 'a level spoonful' (flat across the top), which matter when you are baking.