A root canal is a space within the tooth’s root. Some teeth have 2-3 roots. Within this space is nerve and a blood supply termed the “pulp” which is the soft inner tissue that plays an important role during the tooth's development. Once a tooth is fully mature, the tooth can survive without the pulp because the tooth is nourished by tissues surrounding it and no longer needs to grow or develop. The pulp is normally surrounded and protected by a layer of “dentin” which is protected by the enamel, the whitish outer coating of your tooth that is visible to the eye and provides protection.
When a crack or cavity destroys these protective layers, the pulp is exposed to irritants and bacteria in your mouth. This can result in inflammation which leads to an infection if left unchecked and eventually forms an abscess. Endodontic therapy removes the damaged pulp and usually the tooth returns to a healthy condition. The root canal space is then filled with a rubber based material derived from trees that safely and organically allows the tooth to heal.
If an abscess was present before treatment the healing process may take up to 2 years, this would be better explained by your dental professional.