What does うん mean?
うん is a Japanese interjection meaning “yes (informal), all right (ok)” in English. It can also be translated as “hum, oof” depending on context. It is written as うん in hiragana, and un in romaji. This word is part of JLPT N4 vocabulary.
Example sentence
「コーヒーのお替わりいる？」「うん、ぼくのは少なめ、弟のには多めについでね」
"Do you want another cup of coffee?" "Sure, make mine on the small side but be generous with my brother's."