Numismatic Overhaul adds a currency system to Minecraft, not too dissimilar to how Terraria does it. You have a purse in your Inventory which holds your riches - these may be obtained from killing Pillagers, exploring the world and its ancient ruins and, of course, trading with Villagers.
Speaking of Villagers, these have also been completely reworked. First of all, they obviously trade Currency instead of Emeralds. However, their trades have also been rebalanced and tweaked to provide a more interesting trading experience.
One last feature is the Shop, which serves as an easy way to trade items between players. When opened, it displays a Villager GUI with trades the owner can configure. When someone buys something, the money will be stored within and can be retrieved by the owner.