Often times, the chewing surfaces of back teeth (molars and premolars) have fissures that are deep and narrow.  Such pits and grooves can be difficult to clean.  Sealants are a substance that fill these fissures to prevent the accumulation of food and plaque to seal out bacteria.  Though sealants typically last a few years, they are a much more conservative and proactive treatment, rather than restoring teeth after decay has begun.