Meaning: A thin layer of bacteria and food particles builds up on the teeth, forming plaque.
Choice of Words: "Plaque" is a sticky film that forms on the teeth and contains bacteria and food particles. "Forms" means to come into existence or take shape. "When" indicates a point in time or a condition under which something happens. "Bacteria" are tiny living things that can be harmful or beneficial. "Stick" means to adhere or cling to a surface.