A sealant is a thin layer of "varnish" that protects the occlusal surface (chewing surface) of a tooth. The seal is usually placed on the molars and premolars to protect these teeth from decay.
Studies show that most cavities in children under 15 years old develops on a surface of a tooth occlusal (chewing surface).
These surfaces contain natural furrows where plaque can easily accumulate. This plaque cannot be easily removed even by conscientious brushing. If plaque stays in the groove, a cavity can be formed easily.