Root canal treatment is the process of going inside the pulp space and removing the infected and dead tissue. Then the space is disinfected and sealed with special materials.
Every tooth consists of three different layers. From the outside, first there is Enamel (the outer layer of the teeth which is the hardest part of the body). The second layer is Dentin (the inside hard layer) and then we have the Pulp (which is an empty space where the live tissue and nerve of each tooth is located).
If for any reason the pulp space is exposed to the outside, the tissue becomes contaminated and eventually infected. The exposure of pulp happens in many circumstances for example when you have a large cavity, or a fractured tooth. Your dentist can explain the exact reason for damage to this tissue. Regardless of the reason for this contamination or damage to pulp tissue, the treatment is usually Root canal treatment.
Now-a-days, root canal treatments are performed with advanced techniques and material and are more comfortable and faster. After root canal treatment is done your restorative dentist will usually place a crown on you tooth to protect it against fracture.
Learn more about root canals at American Association of Endodontics