What does "uhm" mean? Does it mean "yes" (impolitely)? And is it a common word?
I could mean 'yes' ('uh-huh'), it could mean 'no' ('uh-uh'), it could mean 'I'm listening' ('uh-huh'), it could mean 'what?' ('huh?'), it could mean 'I'm pausing because I don't know what to say, but I want to keep speaking' ('umm').

All are common, and it is difficult to know which utterance you are talking about without any context.
It most often is what we cause an "articulated pause" -- that is, the speaker stops for a second or two but keeps on making a sound, usually to indicate that he is not through speaking. I have a linguist friend who has made a study of the most common articulated pauses in various languages and has come up with some interesting differences.
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I see the word "uhm" here . But it seems that the author is not serious. By the way, the spelling is un-English, isn't it?
Do not rely upon the Urban Dictionary for any definitions. 'Um' is not a word; it is merely the transcription of a sound, so there is no spelling rule.
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Thank you, Mister Micawber Emotion: smile.
 Philip's reply was promoted to an answer.