There are many situations when you say that. 1. when you are thinking to yourself or thinking about something. 2.when you are puzzled by something or someone or by what they said/did or have done are going to do 3. when you disagree a little with somebody. 4. when you totally disagree 5 when you need a second to compose an answer, but that is usually Hmm it is a very common see here https://www.merriam-webster.com/dictionary/mm-hmm https://www.urbandictionary.com/define.php?term=mmm%20hmm https://www.urbandictionary.com/define.php?term=mmm%20hmm see this discussion for the negative uses https://forum.wordreference.com/threads/uh-huh-or-mmmhmm.332995/ there are many ways to say it and the tone and length of the mm hhhm or uhhh mmmm is important

Hi. In addition to John, the (tone/pitch/rhythm) matters. For example, if you say it fast, then it could mean "yes" or "okay, I understand." Or if you say it slow, then it could mean like number 5 (from John's post).

I feel like humming sound means same thing in every language you use it when you agree w/ someone or just wanna let them know that you heard what they had to say. examples: A: would you like a bag for that? B: mhm, sure! A: and then she just left me in the middle of nowhere! B: mhm, I see..

It can be a simple acknowledgment that you are listening to someone. As if to say 'go on, I'm listening.' But, depending on the inflection, it can also be used sarcastically, or to convey doubt in the person's story. The first is usually short, Mm-hm, the second is usually more drawn out Mmm-hmmmm.