A root canal may be recommended by your dentist when one of your teeth have a very deep cavity.
Inside the tooth lodges the nerves and blood vessels ( also called pulp). When you have a deep cavity, the bacteria may enter the pulp and cause inflammation (pain , swelling).
You may also have a bump on your gum and the nerve may die.
The dentist will anesthetise you (freeze) and then open into the canal and remove the infection, This is followed by placement of an inert material and sealing the opening. The entire process may require one to three appointments. In some cases your dentist may refer you to a specialist. Teeth which have had root canals usually require a crown to be placed on them to prevent fractures.