A sealant is a resin material that is usually applied to the chewing surfaces of the back teeth. A sealant acts as a barrier that protects tooth enamel from bacterial plaque. Sealants prevent cavities
from forming by depriving decay causing bacteria of the food they need to survive.
Because toothbrush bristles cannot reach all the way into the pits and fissures on the teeth that form on the tooth's biting surface, even careful brushing won't remove all food and bacteria from these areas. Sealants protect teeth by "sealing" pits and fissures and creating a barrier against plaque and food.