？
Is it really true?
Literal
True-[attributive-な]-[question-の]?
本当 is a noun / な-adjective meaning 'truth, reality.' When used predicatively before a の (nominalizer / explanatory), it takes な — the attributive form of the copula for な-adjectives and nouns. The trailing の turns this into a soft, curious question. A feminine-leaning or childlike phrasing — casual and intimate.