Reiki is a Japanese word that means "universe energy". As far as I know English lacks a properly equivalent word, because we have this embedded division between physicality and spirituality as a cultural motif in the modern anglophone world, and that impacts our concepts when we express them in our words.
Reiki is everywhere, in everything and in everyone. Usui Ryoho is a method of working with this energy to facilitate healing in the self and in others. This is the origin of most Western Reiki practitioners. However, this is not the only origin, nor is it the only style or method of interacting with reiki.
When Hawayo Takata (who was taught by Chujiro Hayashi, who was taught by Mikao Usui, the founder of Usui Reiki Ryoho) began teaching Reiki to people in the United States of America, she changed several things about the system that downplayed Japanese culture. When US Americans began to teach Reiki from that understanding, they were spreading a Japanese practice without a full awareness of its cultural context; they also charged money for these services and for their classes without proper acknowledgement of Japanese culture.
While this strategy may have preserved Reiki from total loss in the United States of America (because it avoided anti-Japanese discrimination), it unfortunately also separated this healing modality from the spiritual and cultural context of its origin - this is Cultural Appropriation.
As a Reiki practitioner myself, I made sure to take at least one Reiki course offered by a person of Japanese descent and to look for and read books about Reiki that include Japanese sources or are by Japanese authors. In my personal practice of Reiki, I try to honor Japanese culture and the foundations of Usui Ryoho while working from my own energetic cultural experience, ensuring that my energy work is authentic to who I am while respecting the origin of the system into which I was initiated.
Fundamentally, in my opinion, Reiki Ryoho (Reiki Healing Method) is about maintaining a personal relationship with the energy of the world around us, and recognizing our constant, unbreakable interconnection with our community and our environment. What impacts me within, also impacts the world around me, and what impacts the world around me also impacts me within.
In the USA, we tend to approach everything with "vending machine mentality". We want to put our dollar bill into the machine, and take our prepackaged prize and go on about our day. We don't want to stop and have a conversation with the vending machine, or give our precious dollar a going-away-gratitude-party, and we certainly don't pause to appreciate our vended "prize" with reverence for the sacrifice involved in leaving the vending machine to become part of our reality.
And of course, all of that sounds ridiculous when I talk about it in terms of a vending machine.
But the world isn't a vending machine.
Our bodies are not vending machines.
Our spirits are not vending machines.
Reiki is not a vending machine.
You are not a vending machine.
I am not a vending machine.