[overlordalex]

They're considered fillers that mark a continuance on part of the speaker so that the other participants know that they will continue speaking. Different languages not only use different sounds, but in many cases continuation markers can also be "normal" words from the language. In English some examples would be "well", or "yes" which can be pure filler words to bridge to the next utterance.

That being said, what is a word anyways? You could argue that they're well understood units of language that convey meaning, which I would argue is pretty much a word

[philipov]

You make a good point. I looked it up and it's in MW and even scrabble recognizes it as an interjection. I think to be a word, at a minimum an utterance would have to conform to the phonemic structure of a language, but that's a low bar, and 'uh' passes. I guess it's a word!

[eru]

> [...] would have to conform to the phonemic structure of a language [...]

I don't think that's a good criterion, because it would exclude a lot of loanwords.