Your back teeth are known as premolars and molars and have pits and grooves on the biting surface, some of which can be deep and difficult to keep clean with brushing. Food particles can become trapped and this creates a perfect breeding ground for plaque bacteria which if left, will generate cavities. Fissure sealants work by filling the deep pits and grooves making it easier to clean your teeth and thus helping to prevent cavities from forming.