。
She definitely knows about that.
Literal
She [topic-は] certainly that thing [object-を] knows [explanatory-んです].
確かに ('definitely / certainly') is a strong epistemic adverb expressing the speaker's confidence in the claim. ~んです (the explanatory ~のだ in casual contraction) frames the sentence as conveying background or explanation — 'the thing is, she knows it' — rather than just stating a fact. This pairing is common in conversation when the speaker is asserting a contested or emphasized point: 確かに知っているんです reads as 'I'm telling you, she really does know.' Without ~んです, the sentence would feel flatter and more neutral. 知っている (rather than plain 知る) is the standard 'know' form — knowing is treated as a state, not an act, and Japanese always uses ~ている for it.