The grooves on the chewing surfaces of the molar teeth are called "fissures". These fissures can be deep, and very difficult to clean, and at the bottom can be narrower than a single toothbrush bristle. Plaque accumulates at the bottom of these fissures and attacks the tooth enamel, over time cavities can develop.
Dental sealants are plastic coatings that are typically placed into the dental fissures of the back molar and premolar teeth. These coatings are designed to fill in the fissures with a safe material and protect these fissures from decay.
Over time the sealants will be warn away / damaged by regular chewing. Replacement of the sealant may be required every few years.
The downside of dental sealants is that they cover the fissures and prevent your dentist from seeing any early signs of decay within the fissure. This could be a problem if the sealant has leaked and allowed plaque to form beneath it. The sealants also prevent fluoride and other beneficial minerals from entering the tooth surface within the fissure.
Dental sealants are generally not applied to adult teeth, but may be beneficial where deep narrow fissures are present on the molar teeth.