1An informal interjection meaning 'yes' or 'yeah' used to indicate agreement.
「明日行ける？」「うん、行けるよ。」
"Can you go tomorrow?" "Yeah, I can go."
先生：宿題は出した？ 生徒：うん、出しました。
Teacher: Did you hand in the homework? Student: Yes, I did.
うん、そうだね。君の言う通りだ。
Yeah, that's right. You're right.
2An interjection expressing hesitation or thinking (like 'hmm' or 'well').
うーん、それは難しいね。どうしようか。
Hmm, that's difficult. What should we do?
質問を聞いて、彼はしばらく『うん…』と考え込んだ。
Hearing the question, he paused and said 'hmm...' while thinking.
3A groan or moan of pain; a short sound expressing discomfort or strain.
転んで膝を打ち、彼はうっ…うんと呻いた。
He fell and hit his knee, and gave a muffled groan.