A post and core is a dental restoration used to sufficiently build-up tooth

Tooth

Teeth are small, calcified, whitish structures found in the jaws of many vertebrates that are used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are embedded in the Mandible bone or the Maxillary bone and are...

 structure for future restoration with a crown

Crown (dentistry)

A crown is a type of dental restoration which completely caps or encircles a tooth or dental implant. Crowns are often needed when a large cavity threatens the ongoing health of a tooth. They are typically bonded to the tooth using a dental cement. Crowns can be made from many materials, which...

 when there is not enough tooth structure to properly retain the crown, due to loss of tooth structure to either decay or fracture. Post and cores are therefore referred to as foundation restorations.
Post and cores divide into two main groups: prefabricated and cast.