The inner most portion of a tooth is hollow and in this hollow space is the nerve of the tooth.  When a root canal is performed this hollow space is cleaned out and disinfected.  If bacteria from a deep cavity enters this space the nerve  can become painful or infected and may need to be removed.  A small hole is made in the chewing surface of the tooth to access this space and small files are used to remove any remaining nerve tissue while a disinfecting solution is used to cleanse the space.  After thorough cleaning is performed the space is permanently filled to seal it off.  Most often root canal treated teeth are restored back to shape and function with a crown to protect them from breakage.