Root canal therapy, in a nutshell, is the process of removing the infected, injured, &/or necrotic pulp (dead nerve) from the applicable tooth and replacing it with a special type of filling material. After that, a permanent filling or crown should be placed (depending on the condition of the tooth in question).
Saving a tooth is always the preferred option when it is feasible to do so due to problems it can lead to down the road if removed such as: drifting teeth, oral hygiene challenges, and jaw related problems.
The cost of replacing the tooth should also be considered prior to the decision to remove it as it is often more expensive to replace after the fact and the option to replace it may involve compromising other teeth (i.e. in the instance of bridge-work for example).