Root canal treatment - or endodontic treatment to dentists - is the process of removing the pulp (the blood and nerve supply) from an infected tooth and replacing it with filling material.
The pulp of your tooth can become infected if decay passes through the hard parts of the tooth into the root canal system in the middle. This can also occur if the tooth is accidentally damaged. When infection starts it can spread throughout the root canal system inside the tooth, eventually leading to an abscess.
The only other effective treatment option is to extract the tooth. This removes the source of the infection (and if the infection has spread too far this may be the only option). However, it is important to keep your own teeth whenever possible. Delaying treatment also reduces the prospect of success, jeopardizing the chances of saving the tooth at a later stage.