。
What kind of thing is innocence?
Literal
Innocence [quotative-とは] how [possessive-の] thing [explanatory-なのです] [question-か].
無邪気 ('innocent, naive, guileless') — 無 (none) + 邪気 (malice). とは is a definitional quotative: 'as for [X].' A philosophical question — 'what is innocence, really?'