Directly after you finish playing an Action card, you may call this, for +2 Actions.
Coin of the Realm is a Treasure-Reserve from Adventures. While only a Copper on play, it can be called(call) after playing an Action to give +2 Actions, making it a village(Village (card category)). Normally this means having to wait until the turn after playing it to call it, but if played by Black Market or Storyteller, it can be used as a village on that same turn.