Although root fillings do make some people nervous, with modern techniques it is un-likely that you will feel a thing.
A tooth consists of a crown which is the part you can see and a root system with nerves that extend into the jaw bone and hold the tooth in position.
Roots can become infected in a number of ways, for example if a filling leaks or the tooth has been damaged by a fall or by chewing on something hard.
If the nerve in a tooth is damaged due to decay or injury, the tooth can become infected and cause pain. A root filling, also known as endontics, treats the damaged tooth by cleaning out the bacteria in the roots and replacing part of the tooth with a filling.
Once the tooth has been successfully treated it may need to be strengthened by an onlay or crown.
Below from left: 1 The infected tooth, 2 The tooth which has been opened up, 3 The removal of the infected roots, 4 Replacement fillings have been done and a new onlay strengthens the tooth.

Dr Jayme Adams is a Dental Phobia Certified dentist Click Here to verify.