One of the parts of a tooth most susceptible to decay is the area with deep grooves and pits.  They are found on the biting surfaces of back teeth.  It is in these crevices that bacteria and sugar collect and create decay.  This can be prevented by cleaning out these grooves and filling them in with a resin sealant.  The sealant acts as a protective coating that blocks out decay-causing bacteria making the teeth easier to clean.