I've been a Reiki healer since 1982 and a teaching Reiki Master since 1989. The name Reiki means "universal life energy." The Reiki attunements will connect you at sequential levels with this higher life force. I think of it as God's love, although there are other definitions! My own Reiki teaching has an extra attunement that will connect you with the Christ energy (included at His request).
The first level confers the ability to heal yourself and others. The second level adds more power to your healing, and enables you to heal in the mental/emotional areas, as well as to heal at a distance. The Masters level enables you to teach, to pass the energy along to others.