Root Canal Treatment is used when there is an infection at the center of a tooth (the root canal) which is usually caused by bacteria that live in the mouth and invade the tooth. First your dentist will check if you need the treatment by looking at the inside of your teeth with an x-ray to see if the pulp is damaged or not.
A single tooth might have multiple canals and they may need to be treated separately. After the x-ray your dentist will clean the bacteria in the teeth and fill it with a filling or a crown depending on the situation.