Some dentists use devices called "pins or posts" to anchor very large fillings placed in your teeth. The pin or pins are countersunk with a handpiece (dental drill) half way into your tooth outside its nerve chamber (canal) after all decay has been removed.
A dental post is used in dental restoration to sufficiently build-up tooth structure for future restoration with a crown when there is not enough tooth structure to properly retain the crown, due to loss of tooth structure to either decay or fracture