You just have to explain how too little or too much of that nutrient disadvantages or benefits health. You can do this in terms of linking the nutrient to how nutritional deficiencies that may arise if you don't have enough of it (e.g. iron -> anaemia/ Vitamin D -> Rickets / Fluoride -> dental caries / Vitamin A -> night blindness). You don't HAVE to know the deficiency 'names' but at least know the health effect of being deficient in that nutrient (e.g. instead of writing 'dental caries', you could always just say 'prevents teeth decay' so you don't have to be extremely technical in terms of your answer)
Or you can explain the 'health function' generally, in terms of indicating how the nutrient impacts health through its effect on body functions. E.g. sodium's "health function" is to maintain normal functioning of nerves and enable normal muscle contraction
and in terms of the macronutrients, you just do the same so although you need e.g. protein for repair and maintenance of cells, too much of it easily converts to fat which increases risk of atherosclerosis. and although you need for e.g. sodium for normal functioning of muscles and nerves, too much sodium obviously causes your heart to pump blood extremely rapidly thereby causing high blood pressure (hypertension).