Dental sealants are a coating we put in the deep pits and groves in teeth. These pits and grooves continue into the tooth well past what your eye can see and tooth brush can reach. But those tiny little bacteria can get down into those grooves; they can camp out there, have some nice carbohydrate dinner and poop out some acid making tooth decay in those deep pits and grooves. Sealants coat over those grooves sealing them off from the bacteria. No more groove camping bacteria = less chance of tooth decay. Sealants have been shown to be very effective at preventing tooth decay.
We usually recommend sealants for any teeth that have deep pits and grooves but don’t already have fillings. This happens a lot more often in kids because most adults have had time to either wear down their teeth enough not to have as deep of grooves or to get decay in there so that they already have filings. But for those adults that are lucky enough to still have pristine teeth, I would still recommend dental sealants.