A root canal may be needed if decay has reached the tooth root/nerve. Essentially, a root canal cleans out a tooth's infected root canal then filled with a material that acts like a tooth's 'pulp' then sealed.
An opening is made in the crown of the tooth, and the pulp chamber and root canals are cleaned and shaped for filling. The root canals are then filled and sealed with a rubber-like material to ensure complete sealing of the root canals. In most cases, a filling is placed to close the opening, however, in some more severe cases, the tooth will be closed by applying a crown and a buildup.