The belt encircles its wearer. The circle is a universal symbol of wholeness and harmony, and symbolizes the totality of the universe. The circularity of the belt reinforces the circular cycle of training; the fact that, after years of training, one realizes that the true essence of Karate-Do existed at the beginning.
As a practical matter, the belt holds the uniform closed, but its real significance is far greater than merely being a clasp or even a signifier of rank. The belt has symbolic meanings, both in eastern philosophy and in its color.
In eastern philosophy, the concept of trinity (heaven, earth, and people) signifies the harmony of the universe. The parts of the uniform (jacket, pants, and belt) form a trinity. The jacket symbolizes heaven; the pants symbolize earth, and the belt symbolizes the "person" that ties it all together. As stated above, the colors of the belt also form a trinity.
If you think of a human being as a a trinity (consisting of a head, the body, and the extremities) then the body is at the center of a human being, and the waist is at the center of the body. Tying the belt around the waist signifies the desire to organize ones self and to unite the human trinity.
The belt helps students develop their ki/chi (inner energy) through the process of collecting and dispersing energy within their bodies. As the student puts on the belt, it encircles the waist two times and then the two ends meet at the center of the waist (tanden or hara) where they are tied in a triangular shaped (trinity) knot that denotes the oneness of a person.
The tanden, considered the source of ki/life force/vital energy, is a point about three inches below the navel and deep inside the body. it is thought to be the center of the self. As a practical matter, it is also the approximate center of balance of the body. part of the knot usually touches the body in front of the tanden, reminding the wearer of his or her personal source of ki or power.
Thus, while putting on the belt, the student encircles and collects all energy from without and within into the tanden and locks it there with a knot. He or she can disperse the energy freely throughout the body to achieve power, harmony, order, and enlightenment while training.
This information comes to us via Adam's Karate Fitness.