What is dental post? 
A prefabricated post is a metal pin placed in the root of a tooth that is healthy enough to hold it. The natural tooth's root is preserved. A crown or bridge is always affixed to the postPosts are recommended when a root canal has been performed and not enough natural tooth remains to support a restoration.

Why is a post and core needed?
    A post and core is indicated when a large part (or all) of a tooth's original crown structure has            been lost. The crown structure can be damaged either due to the progression of dental decays or        because of a tooth trauma. A post and core can only be manufactured if the tooth has had root          canal treatment.