Emeralds are the main currency in Minecraft Dungeons (other currencies include enchantment points and souls). They are used in the Camp to purchase gear from the Luxury Merchant, Village Merchant and Mystery Merchant. They can also be used to gift items with the Gift Wrapper and upgrade items with the Blacksmith.
Emeralds are dropped by most mobs in the game, in various amounts, and can also be found in chests and emerald pots. Items can also be salvaged for emeralds and enchantment points. After beating Creeper Woods for the first time, you will find one emerald chest near camp that contains 50 emeralds. This will continue until you beat Obsidian Pinnacle.