There are many sites online explaining what Reiki is, so what I’m going to do is offer my personal experience with Reiki.
I came across Reiki when I was studying Holistic Hypnotherapy and Hypnoenergetics, so I was already learning about energy when one of my teachers brought up Reiki. We asked her what it was and she simply pulled my hand out in front of me and held her palm above the top of my hand. I felt a magnetic force and a warmth flowing down from her hand into mine. It was pleasant and soothing, and I wanted to close my eyes and enjoy it.