More than 100 years ago, D.D. Palmer, the founder of Chiropractic, drew a connection between the state of the nervous system and the health of a person. He defined tone as "the normal degree of nerve tension."
All living tissues (and organs and organisms) have tone. It is a quality that can be perceived and felt, and through specific types of biometrics, can be measured. When tone is normal, the function of the tissues and organs is normal, and this is the primary marker for healthy physiology.
The nervous system is what allows us to perceive and respond to the world around us and to coordinate how the body moves, feels, and heals. When there is tension on the nervous system, it is reflected in the other tissues of the body. Since the spine is intimately related with the spinal cord and brainstem, and serves as the communication highway between the brain and the body, nerve tension often shows up in the spine.
Tone can also be seen as an acoustic phenomenon. A stringed instrument, when in tune, requires the strings to exist at certain tension. When energy is added to the strings to make them move, they oscillate within a specific range of frequencies, and the sound is amplified by the body of the instrument. The ability to express the song, to do so with ease, and for there to be harmony in the process requires both structural alignment and tonal refinement.
When the instrument of the human body is not "in tune," it is because tone is not being expressed normally. In Chiropractic, this is referred to as dis-ease. An attuned nervous system is an adaptive nervous system, and one that allows for health, vitality, and empowerment. This is the basic principle. This is what Chiropractic is founded upon.