Root treatment is a branch of dentistry, which deals with the diagnosing, treatment and prevention of the illnesses of the tooth nerve (or tooth pulp) and the root tip area. Often it is the root treatment, which allows to retain the tooth, which otherwise would need to be removed.
Root treatment is needed by a tooth, the nerve of which is infected or dead due to either a very deep cavity or trauma. In such a case the ordinary tooth filling does not solve the problem, as through a root canal without circulation the infection will spread to the jaw, where it can cause even life-threatening complications. Often root treatment requires several visits. As a result of a successful root treatment even an infection, which had reached the jaw, would heal. A tooth with a treated root will work like any other tooth in the future even though it may be more fragile. Often a tooth like that will need to be strengthened with a crown.
Source used: the information page “Root treatment” by the Estonian Dental Association