Hello! nice to meet you! the question is simple but also a comprehensive issue.
「嗯」can have many different meaning, it's kind of exclamative particles, but the word「exclamative particles」(感嘆詞) is a profession word for non-native Chinese learner or used for linguistics. Native speaker often use them in ordinary life but doesn't realize it, but if used them fluently it would be naturally for listener.
Èn, 好吃 → Mm, very tasty = it's a kind of onomatopoeia and just to express that the food is yummy.
Or in some situation, it would have meaning of ''I'm thinking'' or ''I'm considering ''、''I understand'' in different tones.
It's a common exclamative particles.
And here's an example that uses lots of「exclamative particles」:「相聲」、「相声」,it's like Stand-up comedian with one person or two people: