A sound people make when they are thinking or not sure what to say.
The sound 'hm' can show that someone is thinking or not sure.
Examples:
Hm, I'm not sure about that.
Hm, what should I do now?
She said 'hm' and then told me her idea.
Hm, I think I left my toy at home.
Hm, I wonder what's for dinner tonight.
Hm, I need to think about it for a while.