Teeth are living things. They have a hollow center with a nerve and blood supply. For any number of reasons the nerve or blood supply is injured or dies a root canal is called for to restore the tooth.
When the dentist is preforming a root canal procedure he removes the damaged material, shapes and cleans the hollow space within the tooth, and fills it with an inert material.
A root canal basically leaves the tooth lifeless. The body will not reject it unless leakage occurs, the tooth breaks, or new decay develops. Most teeth treated will require a crown coverage for strength especially if is a back tooth. Sometimes a front tooth can be repaired with a tooth colored material.