Sealants are thin layers of resin that are placed on the pits and fissures and grooves of (usually) molars to prevent decay on these surfaces
Majority of decays on back teeth starts on grooves and pits of chewing surfaces, especially during the first few years after their eruption. Sealing these surfaces with composite resins prevents these kinds of decay.
Sealants are one of the most effective methods of preventing decays on the surfaces where they are places. Although it is still a possibility that decay may happen on surfaces in between the teeth, sealants significantly reduce the overall chance of having cavities.