The chewing surfaces (or occlusal surfaces) of the molars have deep fissures into which the bacteria plaque can infiltrate and get trapped. These areas are highly exposed to the risk of tooth decay even when a proper dental hygiene is implemented.
A dental sealant is a is a special white-transparent resin that continously releases fluoride ions, that is made slide into the occlusal fissures, that have been previously treated with an acid to make them porous, and to efficiently stick the sealant, that hardens thanks to the use of halogen lamps, and that, once applied, prevents the bacteria plaque to penetrate inside the fissures.
The sealings are made on the back teeth, premolars and molars, because these are the teeth that usually have fissures and pits on the chewing surface. The specialist will advise or not the sealings: some teeth form deep and narrow fissures that need to be sealed, others instead form less deep fissures, that are not exposed to a high tooth decay risk.
Seals usually last many years, but the specialist will have to check them periodically in order to make sure they are always intact and efficient, because they could wear thin.
Seals should be applied if necessary as soon as fixed teeth start erupting, normally between the ages of 6 and 7. The other teeth can be sealed as soon as they arise, and this may be between the ages of 11 and 14.