Inside your tooth is a system of canals that contain tissue (blood vessels and nerves).  In a normal tooth you use these tissues to detect hot and cold.  When there is trauma to these tissues (usually through decay or removal of decay and subsequent restorations), your body cannot always heal the tissue, and it will begin to deteriorate.  This deterioration can cause immediate pain or set up a situation that may cause pain and infection in the future.  Root canal therapy removes this damaged tissue, prepares, and seals the root canals so bacteria cannot get into the space.