A question formed in the negative (a negative question). It often expresses expectation, surprise, or seeks confirmation (e.g. "Can't you read Japanese?").
まだ宿題をやっていないの？
Haven't you done your homework yet?
日本語が読めないんですか？
You can't read Japanese?
そんなこと、知らないの？（＝知らないのか）
You don't know that?