It is administered by "laying on hands" and is based on the idea that an unseen "life force energy" flows through us and is what causes us to be alive. If one's "life force energy" is low, then we are more likely to get sick or feel stress, and if it is high, we are more capable of being happy and healthy.
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". So Reiki is actually "spiritually guided life force energy."
Read the full story at: The International Center for Reiki Training