The root canal is the space within the root of a tooth, which is filled with pulp. If the pulp of a tooth, which contains the nerves and other tissues becomes diseased or injured and is left untreated it can result in pain and the loss of the tooth.    
Root canal treatment simply means that this space is cleaned out, disinfected and filled to prevent further infection and discomfort. It is performed under local aneasthesia so is no more painful then a filling.