What does "hmm" exactly mean?

Pingju
I am asking because:

1) I have seen the word "hmm" being used quite a number of times as the first word in GAGers' opinions when they answered the questions I posted on this site.

2) I have seen the word "hmm" being used quite often by people whom I was having text conversations with on my phone when I was telling them something important or serious or telling them some observations I had made. Does using "hmm" here mean that they disagreed or were doubting what I had just shared with them?

Thanks for your honest answers.
What does "hmm" exactly mean?
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