The Ender Shard is a component item added by Thaumic Tinkerer. It cannot be crafted and drops rarely from Endermen. It is used in the creation of Ichor.
Similarly to the overworld, the Nether and the End also have theor[sic] specific shards, these are rarely held by other Zombie Pigmen living in the Nether or Endermen living in the End. They might be useful for something later on, although you're not quite sure how.