Reiki is a healing treatment that started in Japan. It is similar to the Christian tradition (if not the same thing as) laying on of the hands.
For myself, I find it a beautiful way to live my faith and allow that God/Universal energy to flow through me to help heal others. I truly know it is not ME that heals, but God (you may choose to think of it as Universal energy). It is a wonderful gift to be able to share, and something I love doing.
If you really want to learn more, I recommend visiting the page at the International Center for Reiki Training for a really comprehensive definition. The link is here: Reiki.org