Everything is energy—everything. Our body is energy. When we feel stress, illness, trauma, etc. our beautiful life energy may become unbalanced, or stuck. If this unbalanced or stagnant, stuck energy is left for a period of time, the result may show up in a form of illness or disease. Reiki is an energy healing modality from a Reiki practitioner using energy flowing through them, to the client. It is important to note this energy is not from the practitioner, rather the energy flows through the the practitioner. The practitioner has learned and become attuned from a Reiki Teacher to accept the flow of energy through them for healing purposes.
Link: Reiki Workshops