These are done mainly for children when their adult molars show up. Occasionally the premolars just in front of them are done.
When adult back teeth form they have grooves in their biting surfaces. Sometimes these grooves are deficient meaning they have not completely sealed together so there is a communication from the mouth through to the dentin layer below the enamel layer. When children come in for checkups these grooves are probed to see if there is any kind of stick at all, meaning they are deep and susceptible to being uncleansible hiding places for bacteria that can invade the dentin layer and become entrenched there as dental decay. The first molars (the 6’s) come in around age 6 years. The next set, the second molars (the 7’s) come in around age 12 years. These are the key ages to check for faulty grooves and do some prevention.
When these grooves are found they are cleaned out and a resin is flowed into them to seal them off from trapping food and bacteria. No anesthetic is necessary. The procedure takes less than 10 minutes and is hugely preventative. Fissure sealants can last for years and often only need to be done once.