1An interjection/adverb used when you understand or agree with something you've just been told — roughly "I see" or "that's right." Very common in spoken Japanese and usually written in kana.
A: このボタンを長押しすると設定メニューが開きます。 B: なるほど、そういう仕組みなんですね。
A: If you long-press this button, the settings menu opens. B: I see — so that's how it works.
彼が留学した理由を聞いて、なるほどと思った。
I heard his reason for studying abroad and thought, "I see."
なるほど、これを使えば問題が簡単に解決できる。
I see — if we use this, the problem can be solved easily.