A root canal treatment is necessary when the pulp inside your tooth gets infected.
The pulp is the soft tissue inside your tooth, home to connective tissues, blood vessels and nerves. Normally, the tooth itself protects the pulp, but if the tooth is damaged by a very deep cavity, or a crack, bacteria can damage the pulp and the mouth becomes painful and sensitive to hot and cold. Not all pulp infections bring pain.

During the root canal treatment your dentist will remove the nerve from the tooth by drilling a hole in the top surface of the tooth removes the pulp.( and with this the infection as well)
He cleans the inside of the root, applies antibiotics if it is necessary When the root canal is dried , he places a filling into the inside of the tooth.