A fissure is a dent on tooth surface which is used for chewing. It is a space so narrow that toothbrush fibers cannot reach it, but food remains and bacteria can. This is how teeth are damaged.
There is more than one material for sealing fissures, but they are all similar to the ones used for fillings. That material is glued to the enamel and lasts for several years. Fissure sealing prevents bacteria from stockpiling and reduces the risk of teeth damage up to 70%.