Piety can refer to either religious devotion, spirituality, or a combination of both.
The word piety comes from the Latin word pietas, the noun form of the adjective pius (which means "devout" or "good"). Pietas in traditional Latin usage expressed a complex, highly valued Roman virtue; a man with pietas respected his responsibilities to other people, gods and entities (such as the state), and understood his place in society with respect to others. The Latin term in turn may derive from "Piodasses", an ancient Greek transliteration of the Indic Prakrit term "Piyadasi", meaning "beloved of the Gods". 
Similar to Benevolence Leves, Piety requires you to deplete either 4 or 8 nodes, each of which have two items unique to the leve. Getting a high rating (dependant on how much XP you get from each item you gather) earns you either a 5%, 10% or 25% exp bonus.