REI (pron. "rey") means present everywhere, or Universal. Another meaning is Spiritual Wisdom or Higher Knowledge. The Rei portion means the spiritual wisdom, which knows how to bring about healing.
KI (pron. "key") means life force energy. Anything that is alive has an energy field with this energy moving through it and around it. In India this same energy is called prana, in China it is called chi. This life force energy is in every cell of yoru body and supports the biological and chemical components to sustain health and well being.
Daily stress from living breaks down our energy field, leaving us vulnerable to disease and illness. Reiki is a process that allows the Universal energy to fill the aura of the body to bring it back into balance.
Some individuals are more sensitive than others. Some things you may experience are:
I, myself have experienced different colors and patterns with my eyes closed, twitching, emotional release, falling asleep, release of blocks. Many times after a session, I have felt like a new person with increased energy, awareness and clarity. Everyone's experience is different.