Teeth are held in the jaws by their roots. Front teeth normally have one root, but teeth further to the back have more.
At the core of each tooth is a soft mass of tissue called the pulp. In a healthy tooth, the pulp contains living fibres, cells, nerves and a blood supply, extending into the roots through the canals.
Decay or injury can destroy the living pulp. Because dead pulp lacks a blood supply, it is more prone to infection, leading to an abcess and toothache.
Root canal treatment means removing damaged or dead pulp and filling the space that is left. The remaining tooth can then be repaired.