The surfaces of your molars and premolars have pits and grooves called fissures. If bacteria gets into these areas it can produce the acid that causes tooth decay. Tooth fissures can be difficult to get into with a toothbrush. Which is why we use fissure sealants to seal them up.
The sealant forms a plastic coating over the fissures to prevent plaque and acids getting in. And hey presto, you’re smiling!