Source: American Dental Association
A sealant is a plastic material that is usually applied to the chewing surfaces of the back teeth-premolars and molars. This plastic resin bonds into the depressions and grooves (pits and fissures) of the chewing surfaces of back teeth. The sealant acts as a barrier, protecting enamel from plaque and acids.
Thorough brushing and flossing help remove food particles and plaque from smooth surfaces of teeth. But toothbrush bristles cannot reach all the way into the depressions and grooves to extract food and plaque. Sealants protect these vulnerable areas by "sealing out" plaque and food.