Reiki is energy from the universe. Much like how we talked about how everything in the universe runs on a vibration and everything is energy- Reiki is energy and it comes from the universe.
When you get attuned to Reiki, it opens your energy up to being a channel for Reiki. You receive Reiki and it flows through you. This means that you can channel Reiki through you, to heal yourself and others. Using energy from the universe, or Reiki, you can heal just about anything from physical, energetic/spiritual to emotional issues. I like to think of Reiki as a great addition to any treatment plan you already have going from your physician.