This shield is a Relic of Saint Vitruvio

This shield has 3 charges, which you regain each day at dawn. When a spellcaster within 60 feet of you targets you with a spell or includes you in the area of a spell, you may expend a charge from the shield to cast counterspell as a reaction. 

If attuned to 3 or more Relics of Saint Vitruvio you gain the Power of Saint Vitruvio feat.

*Credit Monty Martin and Kelly McLaughlin via Dungeons of Drakkenheim

Shields require the Utilize action to Don or Doff. You gain the Armor Class benefit of a Shield only if you have training with it.