A Reiki exchange, Reiki share or Reiki circle is usually a gathering of Reiki practitioners who come together to practice Reiki on each other. The exchange may include a meditation, a distant Reiki session for a specific person or event, conversation around healing topics and similar ideas, a presentation of a technique or idea, etc. 

But mainly it is a place to for Reiki practitioners to practice giving sessions to each.  Generally, several people apply Reiki to one person at a time.  Thus, each person takes a turn receiving a powerful 15 to 20 minute session.  Since a Reiki practitioner receives Reiki as she or he applies it, obviously the evening can be a very strong healing experience.