Memory Fragments were first created when Es separated herself from her memories out of despair. The appearance of these Memory Fragments transformed the World of the Tsukuyomi into the Phantom Field. Additional Memory Fragments are created based on the memories of those who enter the Phantom Field and those close to them. Memory Fragments are necessary to pass the barriers between the floors of the Phantom Field. When one travels between floors, they view the memories inside the Memory Fragments.
In order to reach Little Sister at the bottom of the Phantom Field, Me collects Es's Memory Fragments. In addition, upon reaching the bottom floor, Me finds a Memory Fragment based on Father's memories of her and Little Sister's births.
A crystallized fragment of someone's memories. If you have possession of one of these fragments while moving to a lower level of the Phantom Field, you can view the memory encased within it. 
- XBlaze Lost: Memories, TIPS, #18