Teeth begin to develop in infancy and continue to develop into teenage years. The architecture of a tooth contain "hills" and "valleys" the valleys of the teeth don't fully seal until your later teenage years. During this time all the food that can cause cavities is in contact with the weaker tooth structure of unsealed valleys, during this time the tooth is more likely to get a cavity. This is why sealants are recommended at the time the 1st molars erupt in order to protect the tooth from getting unwanted cavities.
Dental sealants act as a barrier, protecting the teeth against decay-causing bacteria. The sealants are usually applied to the chewing surfaces of the back teeth (premolars and molars) where decay occurs most often. A sealant is just like a composite filling that is more flowable which is able to flow into valleys of the tooth.