。
She's completely naive about the world.
Literal
She [topic-は] completely world-naive is.
世間知らず (せけんしらず) means 'naive about the world' or 'sheltered' — literally 世間 (the world, society) + 知らず (not knowing). The ~ず ending is the classical negative form of ~ない, still alive in set compounds like 知らず知らず ('unknowingly'). Here, 全く means 'completely/utterly' — its affirmative sense, distinct from the 'not at all' meaning it takes when paired with a negative predicate.