Teeth when in jaw are alive like any other organ and need nutrition which is supplied by blood vessels. These vessels are minuscule, are confined in inner portion of tooth and is called as "Dental Pulp".
Also there are nerve fibres in this pulp which cause pain and sensation when stimulated. The space taken up by this pulp is called as "Root Canal" as its canal like in shape and extends from root tip to upper part of the teeth.
When any infection reaches near or inside the dental pulp by any reason be it cavities or trauma or some other reason, it causes pain and discomfort. This needs to be treated and the treatment is called as "Root Canal Treatment" wherein the infected pulp and nerves are removed from the teeth and the empty space is thoroughly cleaned by fine instruments. Once the infection is eradicated the tooth is filled with special filling material. Further for giving strength to the teeth, patient is advised to get tooth fully covered by strong material in form of crowns.