Fillings are done to remove decay, and replace the affected tooth structure. It is called a filling because new a material fills hole that decay left. Most teeth are treated with bonded tooth colored composite resin fillings. Resins are glued to the tooth, and enable most people to go longer before having a crown put onto their tooth. It also decreases the amount of mercury released into the environment, as it is released when the old fillings are cut out of the tooth.
Caught early enough, cavities can be treated easily and painlessly, and the fillings are kept smaller using magnification and resin materials. If not treated decay can lead to tooth pain and/or infection, and the tooth would need root canal treatment or extraction.