Reiki is a form of alternative therapy commonly referred to as energy healing. It emerged in Japan in the late 1800s and has been adapted into varying cultural traditions across the world. Here are some key points to understand about Reiki:

Energy Healing: Reiki practitioners use a technique called palm healing or hands-on healing through which a "universal energy" is said to be transferred through the palms of the practitioner to the patient in order to encourage emotional or physical healing.

Holistic Approach: Reiki is a holistic practice. It is used to treat the whole person including body, emotions, mind, and spirit, creating many beneficial effects that include relaxation, feelings of peace, security, and wellbeing.

Non-Invasive: Reiki is non-invasive and is often considered safe. It does not involve physical manipulation or the ingestion or application of any substances.

Complementary Therapy: While Reiki is not a substitute for medical treatment, it is often used as a complement to traditional therapy and is practiced in many hospitals and medical care settings.

Spiritual, Not Religious: Reiki is spiritual in nature, but it is not a religion. There is no dogma, and there is nothing you must believe in order to learn and use Reiki.

Regarding the different levels of Reiki practitioners, there are generally three or four levels of mastery:

Level I: This is the beginner level. A practitioner at this level can perform healing on themselves and others but only by direct touch.

Level II: At this level, practitioners can perform Reiki from a distance. This level involves the use of symbols and can include sending energy across time and space.

Level III/IIIa (Master Practitioner): This level is for those who want to further deepen their practice. They learn more advanced Reiki techniques and symbols.

Level IIIb (Master Teacher): This is the highest level of Reiki. At this level, a practitioner is considered capable of teaching and attuning others to Reiki energy.

It's important to approach Reiki with an open mind but also with a critical eye, especially when it comes to claims of medical efficacy. While many people report positive experiences with Reiki in terms of relaxation and stress relief, its effectiveness for treating medical conditions is still a topic of research and debate in the scientific community.