* Indecision or uncertainty: "Hmmm, I'm not sure if I should go to the party."
* Thoughtfulness or consideration: "Hmmm, that's an interesting idea."
* Skepticism or doubt: "Hmmm, I'm not convinced that's true."
* Disagreement or disapproval: "Hmmm, I don't think that's a good idea."
* Curiosity or interest: "Hmmm, tell me more about that."
* Simple acknowledgement or agreement: "Hmmm, I see."
The meaning of "hmmm" can be influenced by the context, tone of voice, and other nonverbal cues.
It's basically an interjection that expresses a wide range of emotions and thoughts.