Normally the back teeth, especially molars, have deep grooves which are known as pits & fissures. These pits & fissures are generally deep in kid’s teeth but with age become shallow. It can be difficult to remove all food particles from these depressions and grooves. Hence the food gets stuck more in kid’s teeth leading to decay. 
This situation can be prevented by placing fillings known as pit & fissure sealants. The purpose of sealants is not to replace the oral care routine of twice-daily tooth brushing and daily flossing but to provide an extra layer of protection to these deep depressions on teeth. 
Sealants usually last for several years before you need a reapplication, but you should still visit your dentist regularly for a professional cleaning and evaluation to ensure good oral health.
