Fragment of Reminiscence (追想の欠片) is a special item.
How to Obtain
Fragments are used to unlock past Event stories in the Archives.
Normally, players can only view stories for Events where they have completed all Story Quests. Uncompleted events still appear in the archive, but the event stories are locked.
Fragments can be used to unlock these stories in the Archive menu. One Fragment unlocks all chapters of one Event.