Cavity in a tooth can be fixed with a filling but when it becomes so deep that it involves nerves, gets you severe pain, or even sometimes pus and abscess. Root canals are hollow chambers in the inner core of a tooth. Nerves and blood vessels known as pulp occupy this hollow space and when it gets infected it leads to pain and/or abscess. Root canal treatment involves cleaning of the infected canals of root(s) and filling of decontaminated canals with biocompatible material.