A root canal is a very small, thin division that branches off from the top pulp tissue chamber down to the tip of the tooth root. A tooth has at least one, but no more than four root canals. Root canals are needed when the pulp becomes infected due to a deep cavity or fracture that allows bacteria to seep in. Pain in the tooth is commonly felt when biting down, and also when consuming hot or cold food/drinks.
Without treatment, the infection will spread and the tooth may fall out. Pain usually worsens until one is forced to seek emergency dental attention. During a root canal procedure, a dentist will clean out the diseased pulp and reshape the canal. They will then fill the canal with rubberlike substance to prevent recontamination of the tooth. The tooth is then permanently sealed, with possibly a post and/or crown so that the patient can keep the original tooth.
Source: Academy of General Denistry