You can modify a vast number of objects in path of exile using the various orbs, scrolls, essences, whetstones and other stacking consumables. Collectively these items are referred to as currency, as they are also used to trade and barter with both npcs and other players.
Poe currency piece i am most excited about when i see one drop. Exalted orbs are used to add one random affix to a rare item. They are used for high level crafting and also are the main currency for trading very high value items. If you find exalted orb while levelling, do not use it on random items, this orb is very valuable and can buy you good items for late game.