Teeth have canals inside them that have nerves and blood supply to the tooth. When teeth get traumatized by either repeated cavities, fillings, and crowns, or by actual physical trauma like an accident, the nerve can start to die which can cause pain. The nerve pain can be pain from temperature change, pain on biting, or just an ache. The process of a root canal is to access the nerve canal through the tooth, remove the dead or dying nerve, and seal it closed, so no pain can return.
Root canals have a very bad reputation for being painful, and usually, they are very comfortable procedures. The important thing is to get the inflamed tooth nice and numb, then many patients actually fall asleep during the root canal process. If the tooth being root canalled is not crowned, a crown is often recommended because the access in the tooth to perform the root canal often leaves the tooth weakened. Dr. Bliss will be happy to diagnose the proper treatment for pain that you are having.