How does Reiki work? 
Energy is everywhere and makes up everything. It cannot be destroyed or created. It knows no geographic location and is not bound by time and space.  Awareness and intention direct this energy.  Much like your thoughts put energy into motion= Emotion.
 
Consider the negative and positve charge of a battery. One polarity can not direct the energy without the other opposing polarity. A lightening cloud is the positive charge seeking the negative striking point of the ground. This is the life force energy that surrounds us all. 
A Reiki Master has been attuned in directing this energy.
What you can expect from a Reiki session?
Clearing of physical, emotional and spiritual blockages
Boost clarity and energy
Relieve tension and reduce worry
Improve quality of life
Feel calmer, optimistic and empowered