A post and core is a dental restoration which offers support to the tooth to either stabilize a weakened tooth or provide an anchor for your crown. It is mostly done after a tooth has had a root canal treatment and has very little tooth structure left to support crown.
A small rod like restoration (called a post) is cemented to the root canal inside the root of the tooth. This post protrudes out of the root and a core (filling) is built around it. Sometimes post and core is casted together as one piece. The crown sets on the core.