The outer part of the tooth called the enamel is considered to be the hardest part of the body. However, during eating when teeth breakdown food, some food debris are left on the surface of the tooth and some are swallowed and digested. When these food debris are left on tooth surfaces for a longer period of time, bacteria may grow which may produce acids that can erode the enamel surface. When the enamel collapses, a cavity is created. If the affected area does not reach the nerve of the tooth this can normally be restored using composite materials (tooth-colored restorations) or amalgam restorations (silver fillings). Your dentist will tell you which is best for you.
Please seek professional dental advice regarding this matter.