Root canal therapy or endodontic treatment consists of the removal of inflamed or infected pulp tissue (dental nerve) and the cleansing and sealing of the internal tooth structure.
Root canal treatment may be necessary in cases where there is advanced decay, trauma to the tooth, periodontal disease, bruxism (teeth grinding) or cracked teeth

The purpose of the RCT procedure is to clean and shape the canals, removing all debris and bacteria. 

Unfortunately, removing the nerve and blood supply from a tooth results in it becoming brittle.  RCT is therefore always followed by a cast restoration, i.e. a crown or overlay.  The success rate of the treatment is between 90% and 95%.