All healthy teeth have within them soft tissue containing a blood supply to cells, nerves and living fibres. This tissue (pulp) within the core of a tooth extends down the tooth's roots via root canals.
Trauma or decay in a tooth can damage the blood supply to this living tissue leaving if prone to infection. In order to save the tooth this dead pulp must be removed from the canals. The dentist uses narrow files to remove the pulp from the canal of each root. Front teeth just have one root and canal but the back teeth will have more.
Once the canals have been cleaned and free from infected tissue they can be filled up (root canal filling), the rest of the visible tooth above the gum can then be restored. As a root treated tooth has no blood supply and is 'dead' it may discolour and be more brittle than a healthy tooth. A crown is recommended to restore and protect the tooth.