This shard was a portion of a magical artifact, thought unbreakable but shattered by even more powerful magic. The shard still holds some of the item's original enchantment, and can be used to augment the Shield spell.
When you cast Shield and use the shard as a spell component, you create a solid barrier you can use for cover, but not for blocking. You can't use the Shield Block reaction when the spell is modified this way, but you (and only you) can spend an action to Take Cover and gain standard cover from it.