Reiki is a technique that is passed on from teacher to student over and over. If one has Reiki, than they will be part of a succession of teachers leading back to the founder of the system of Reiki one is practicing.
A Reiki lineage is a list of Reiki Masters (teachers) starting with the person that taught you Reiki and then going back, teacher by teacher, to the founder of Reiki. In the case of Usui Reiki, the lineage would lead back to Dr. Mikao Usui. My Reiki Lineage is provided for you below; please let me know your questions.