Dentist recommends root canal therapy on that tooth. what is root canal therapy?

Sealing off roots: A root canal is a procedure when the pulp tissue (nerve and blood vessels) are cleaned out of the root can system inside a tooth, then shaped and sealed off. The nerve tissue was important in the formation of the tooth but not really needed as an adult. This procedure is preformed on teeth that have dead or dying nerve tissue which may or not be infected, may or may not be causing you pain.

Removes the nerve: A root canal removes the nerve, blood vessels, lymphatics and other dead cells in the center of the tooth. After the root canal is done, the tooth will not be sensitive to hot, cold or sweets. The tooth needs a root canal because the "nerve" of the tooth has died, liquefied and drained to the tip of the root can has attracted bacteria.