also: うむ · ううむ
interjection
Casual affirmative interjection, like nodding 'yeah' or 'uh-huh'. Used in informal conversation to show agreement or that you are listening.
「明日、映画に行かない？」「うん、いいよ。」
"Want to go to a movie tomorrow?" "Yeah, sure."
うん、それでいいと思
Yeah, I think that's fine.
interjection
Hesitation or thinking sound, like 'hmm' or 'well...'. Used when pausing to consider something or when slightly unsure.
うーん、ちょっと難しい質問だな。
Hmm, that's a tough question.
「これで大丈夫かな？」「うん…もう少し考えたほうがいいかも。」
"Is this okay?" "Well... maybe we should think about it a bit more."
A natural vocalization; the affirmative sense likely developed from a grunt of acknowledgment. The hesitation sense is an extension of the same sound used while thinking.