If the nerve of a tooth is irreversibly damaged from decay or trauma, it must be removed.  The choice then is to either remove the entire tooth, or remove just the nerve and keep the tooth.  A root canal does just that:  it removes the unsaveable nerve, disinfects the inside of the tooth, and seals it shut with a watertight filling to keep out bacteria.

The root canal procedure has changed dramatically for the better over the last 20 years.  It typically is a no more difficult experience for the patient than that of having a filling.