There are typically three (sometimes four) levels of reiki training—each one is centered on “attunement” (a ceremony of empowerment), education, and practice.
Attunement is what makes reiki unique from other forms of healing touch and energy work. This ceremony, called Reiju (Ray-joo) in Japanese, is performed by the reiki master during each level to open and expand the main energy channels of the student’s body, allowing universal energy, also known as Qi/Chi/Ki, to flow more freely and deeply through them. It tends to clear blockages in the body and can lead to significant energy changes within the individual. Many will undergo an intense period of self-growth following their attunements. You can think of the attunement process like a transfer of energy: The master is passing energy off to the student, who will then possess it for life.
Here is a more detailed breakdown of the three levels of reiki and each of the attunements: