なるほど (naruhodo) means “I see” | MotiTan Japanese Dictionary

More examples

なるほど、そういう意味だったのですね。na ru ho do sō i u i mi da tta no de su ne

I see, so that makes sense now.

Signals that something has become understandable and convincing, often with a sense of agreement or recognition.

ああā: Marks sudden recognition of information or an idea.
そうですねsōdesune: Expresses agreement after understanding what has been said.

相手の説明や事情を理解し、納得した気持ちを表す言葉。ai te no setsu mei ya ji jō o ri kai shi nat toku shi ta ki mo chi o arawa su koto ba

The expression なるほど (naruhodo) comes from 成る (naru, "become") and 程 (hodo, "extent"); its earlier literal sense was "to that extent."

In conversation, なるほど (naruhodo) can show either new understanding or agreement, so its force depends on the speaker's response to the information.