What does “Uh(m)” mean?
Professional linguists from a selection of highly respected international academic institutions have pondered the question in some detail over the years, but now a new paper from Emanuel A. Schegloff, Professor Emeritus at the Department of Sociology, University of California, Los Angeles, goes considerably further towards consolidating the answer(s).
As the professor explains :

“The underlying theme is that accounts for what gets done and gets understood in talk-in-interaction must take into account not only its composition, but also its position—not only with respect to the grammar of sentences, but also with respect to the organization of turns at talk, of action sequences encompassing multiple turns at talk, and of occasions of talk, all of which are demonstrably oriented to by speakers in their production of the talk and by recipients in their analyzing of the talk.”

It has been noted that “Uh”s and/or “Uhm”s in linguistic, cultural and situational settings can perform varying functions not only according to their timing and their intonation, but also dependent on their position in a sentence. The professor clarifies again –

“In adding to the ‘what’-ness of ‘uh’ and ‘uhm’ (the ‘composition’ element) a ‘where’-ness or positional element, we need to go beyond syntactic structure and speech-production processes; it turns out that the conversationally sequential is inescapably consequential for understanding what ‘uh’ and ‘uhm’ are being used to do, both by co-participants and by investigators.“

Pointing out too that the way in which listeners are able (by and large) to assign reasonably accurate meanings to “Uh(m)s” on-the-fly in a normal conversation despite this highly complex possibility-set is impressive – and adding : “Academic work is a piece of cake by comparison”.

The paper Some Other “Uh(m)”s is published in the journal Discourse Processes, Volume 47, Issue 2, 2010, (the Official Journal of the Society for Text&Discourse).

Note : Professor Schegloff provides some audio examples of “Uh(m)s” on this dedicated webpage. ( Needs Flash® – click the individual links below the player)