The word Reiki is made of two Japanese words -
Rei which means "God's Wisdom or the Higher Power"
and Ki which is "life force energy".
Reiki is "spiritually guided life force energy."
It is a Japanese technique for relaxation and stress reduction that also promotes healing. If our "life force energy" is low then we are more prone to get sick or experience stress; if it is high we are more capable of achieving health and happiness.

Reiki is a natural method of spiritual healing and self-improvement that is safe for everyone. It has been effective in helping virtually every known illness and malady, and always creates a beneficial effect.
It also works in conjunction with all other medical or therapeutic techniques to relieve side effects and promote recovery.

***continued...

How does Reiki work?

We are alive because life force / energy / or prana is flowing through us. Life force flows within the physical body through pathways called chakras, meridians and nadis. It also flows around us in a field of energy called the aura. Life force nourishes the organs and cells of the body, supporting them in their vital functions. When this flow of life force is disrupted, it causes diminished function in one or more regions of the physical body.

The life force is responsive to thoughts and feelings. It becomes disrupted when we accept, either consciously or unconsciously, negative thoughts or feelings about ourselves. These negative thoughts and feelings attach themselves to the energy field and cause a disruption in the flow of life force. This diminishes the vital function of the organs and cells of the physical body.

Reiki heals by flowing through the affected parts of the energy field and charging them with positive energy. It raises the vibratory level of the energy field in and around the physical body where the negative thoughts and feelings are attached. This causes the negative energy to break apart and fall away. In so doing, Reiki clears, straightens and heals the energy pathways, thus allowing the life force to flow in a healthy and natural way.

Because Reiki is guided by the God-consciousness, it can never do harm. It always knows what a person needs and will adjust itself to create the effect that is appropriate for them. One never need worry about whether to give Reiki or not. It is always helpful.

*The above text is excerpted from Reiki, the Healing Touch by William Lee Rand.