The mention of Reiki Attunement can sound pretty mystical and the use of symbols might seem strange to people unfamiliar with Reiki. Receiving attunements can feel pretty magical as you become very relaxed and peaceful like there isn't a care in the world.
At each level the Reiki practitioner is attuned to different frequencies. Each frequency is represented by a symbol that has a name. This is very much like a musical note. When you practice listening for the sound frequency of the note A and you practice making the sound on an instrument and reading the symbolic notation for the A, you become better and better able to recognize it and transmit it when it is appropriate.
When a tuning fork, let's stick with the note A, is struck and begins to vibrate with sound, if there is another tuning fork, in A, the room, the sound waves will vibrate and create resonance with the other fork and it will start to vibrate as well. This is basic a physics concept of resonance.
In an attunement the Reiki master holds a certain frequency (Like a state of mind, relaxation, well being, etc.) The Master Teacher will hold that frequency for the student for a length of time and with a level of strength and intensity that they begin to resonate with that feeling in a way that they can recognize the feeling after learning its name and symbol just like a musical note. Then the student can access that energy as desired. The more they practice, the more finely tuned instrument of healing they become.
Why the symbols? Why the names? It gives us a way to communicate about them and more importantly, it gives us something to do with our mind. When we become distracted, as all humans do, we can set our minds on the task of drawing the symbol and thinking of the name of the energy we are using.