Coins are the currency used outside of missions. This currency can be traded between players (?) and used at the Tavernkeep.
The currency Coins has replaced the currency Mana from the previous Dungeon Defenders. Coins can be obtained through killing enemies or selling gear.
Coins can be used to purchase different items from the Tavernkeep. It can be used to purchase:
Cosmetics - Coins can be used to purchase the Invisible Cosmetic Skins which can be used in Transmogrification to make equipment invisible.
Mission Consumables - Items that can be used during missions to give buffs to you, your defenses or to deal damage to enemies.
Miscellaneous - Dyes, which can be used to change the colour of your items and a Respecialization Potion which refunds you all of the skill points that you have gained from levelling up.