Endodontics is the dental specialty dealing with the treatment of the dental pulp. A common procedure performed is Root Canel Filling.
When the nerve in the tooth dies or is dying, in order to save the tooth it needs to be root filled. This is where the nerve is removed from the tooth, the canal widened and measured and a special filling is then placed into the canal to seal it. A filling is then placed over the top.
Because the tooth has no nerve it can become brittle and more prone to fracture so it is advised to a crown (and possible a post into the root for extra retention).