Speech disfluency
A speech disfluency, also spelled speech dysfluency, is any of various breaks, irregularities, or non-lexical vocables which occur within the flow of otherwise fluent speech. These include "false starts", i.e. words and sentences that are cut off mid-utterance; phrases that are restarted or repeated and repeated syllables; "fillers", i.e. grunts or non-lexical utterances such as "huh", "uh", "erm", "um", "well", "so", "like", and "hmm"; and "repaired" utterances, i.e. instances of speakers correcting their own slips of the tongue or mispronunciations (before anyone else gets a chance to). "Huh" is claimed to be a universal syllable.

Fillers
Fillers are parts of speech which are not generally recognized as purposeful or containing formal meaning, usually expressed as pauses such as "uh", "like" and "er", but also extending to repairs ("He was wearing a black—uh, I mean a blue, a blue shirt"), and articulation problems such as stuttering.  Use is normally frowned upon in mass media such as news reports or films, but they occur regularly in everyday conversation, sometimes representing upwards of 20% of "words" in conversation. Fillers can also be used as a pause for thought ("I arrived at, um—3 oclock"), and when used in this function are called hesitation markers or planners.

Language-dependence
Research in computational linguistics has revealed a correlation between native language and patterns of disfluencies in spontaneously uttered speech. Besides that research, there are other subjective accounts reported by individuals.
According to one commentator, Americans use pauses such as "um" or "em", the Irish commonly use the pause "em", the British say "uh" or "eh", the French use "euh", the Germans say "äh" (pronounced eh or er), the Dutch use "eh", Japanese use "ā", "anō" or "ēto", the Spanish say "ehhh" (also used in Hebrew) and "como" (normally meaning like), and Latin Americans but not the Spanish use "este" (normally meaning this). Besides "er" and "uh", the Portuguese use "hã" or "é".
In Mandarin, "那个(nà gè)" and "这个(zhè ge)" are used, meaning "that" or "this", respectively. Arabic speakers say "يعني", the pronunciation of which is close to "yaani", [jæʕni] or [jaʕni], (literally he means, there is no grammatical gender-neutral third person) and Turkish say "şey" in addition to "yani" (without the [ʕ] found in Arabic) and "ııı".Despite the differences between languages, pause fillers in different languages often sound similar because they tend to be the easiest and most neutral vowel sounds to make (such as the schwa), i.e the sounds that can be pronounced with a relaxed tongue or jaw.

Research
Recent linguistic research has suggested that non-pathological disfluencies may contain a variety of meaning; the frequency of "uh" and "um" in English is often reflective of a speakers alertness or emotional state. Some have hypothesized that the time of an "uh" or "um" is used for the planning of future words; other researchers have suggested that they are actually to be understood as full-fledged function words rather than accidents, indicating a delay of variable time in which the speaker wishes to pause without voluntarily yielding control of the dialogue. There is some debate as to whether to consider them a form of noise or as a meaning-filled part of language.
Speech disfluencies have also become important in recent years with the advent of speech-to-text programs and other attempts at enabling computers to make sense of human speech.

"Hmm"
Hmm is an exclamation (an emphatic interjection) typically used to express reflection, uncertainty,  thoughtful absorption, or hesitation. Hmm is technically categorized as an interjection, like um, huh, ouch, erm, and wow. The first h-sound is a mimic for breathing out, and the second m-sound, since the mouth is closed, is representing that the person is not currently sure what to say ("erm" and "um" are used similarly). The pause filler indicates that the person is temporarily speechless, but still engaged in thought. The variety of tones, pitches, and lengths used add nuances in meaning.

Etymology
The expression is used in many different languages, however the origin of "hmm" is difficult to find, mainly because "the word is so natural that it may have arisen at any time," as highlighted by linguist at the University of Minnesota and an expert on word origins, Anatoly Liberman. It is possible Neanderthals might have used "hmm". Nicholas Christenfeld, a psychologist at the University of California, San Diego, and an expert on filled pauses, attests "hmm" is popular largely since its such a neutral sound and that "its easier to say than anything else". The earliest attestations of "hmm" are from Shakespeare, "I cried hum... But markt him not a word" (1598 Shakespeare Henry IV, Pt. 1 iii. i. 154). It may be a vocable that grew out of lexicalized throat-clearing.

Use as a filler word
"Hmm" is a "filler" word, such as "um" and "er". Use of "hmm" for "filled pauses" has been considered by many as stupidity and showing a lack of skill or competence, but many linguists  attest this judgement is unjustified. Typically, "hmm" is uttered when the person is being especially conscious about whom they are talking with, and as a result are thinking deeply about what to say. Moreover, the use of "hmm" is often interactional and cognitive. The interactional function is to do with politeness: if someone is invited to a party and responded "no" without a filled pause, they might appear rude, but a reply of "Hmm, sorry, no" might appear much more polite, as it seems the speaker is giving the offer some thought, rather than abruptly declining.

Thoughtful absorption
The use of "hmm" is typically used during "thoughtful absorption", which is when one is engrossed in their flow of ideas and associations, that lead to a reality-oriented conclusion. The utterance of "hmm" is key for listeners to understand that the speaker is currently engaged in thought; if the speaker thought silently instead, listeners may be unsure if the speaker had finished their utterance. "Um" and "er" are also used during thoughtful absorption; however, typically the extent of the absorption of thought is more limited since "um" and "er" are usually spoken mid-sentence and for shorter periods of time than "hmm". For this reason, thoughtful absorption is typically associated with the utterance of "hmm".

"Huh" – the universal syllable
Research has shown that the word/syllable "huh" is perhaps the most recognized syllable throughout the world.  It is an interrogative. This crosses geography, language, cultures and nationalities.

See also
References
Further reading
Clark, HH.; Fox Tree, JE. (May 2002). "Using uh and um in spontaneous speaking" (PDF). Cognition. 84 (1): 73–111. CiteSeerX 10.1.1.5.7958. doi:10.1016/S0010-0277(02)00017-3. PMID 12062148. S2CID 37642332. Archived from the original (PDF) on 2013-11-10.
Corley, Martin; Stewart, Oliver W. (2008). "Hesitation Disfluencies in Spontaneous Speech: The Meaning of Um" (PDF). Language and Linguistics Compass. 2 (4): 589–602. doi:10.1111/j.1749-818X.2008.00068.x. hdl:20.500.11820/0e5f2f2f-7383-42c5-a7ba-63f2587ad877. ISSN 1749-818X.
Daniel, Ari (5 Feb 2015). "Are we witnessing the death of uh? Um, maybe—and not just in English".
Eklund, Robert (2004). Disfluency in Swedish human-human and human-machine travel booking dialogues (PDF) (Thesis). Linköping Studies in Science and Technology Dissertation No. 882 (Corrected ed.). Department of Computer and Information Science, Linköping University. ISBN 91-7373-966-9. ISSN 0345-7524. OCLC 940753621. Retrieved 11 August 2021.
Erard, Michael (2008). Um...: Slips, Stumbles, and Verbal Blunders, and What They Mean. Anchor. ISBN 978-1-4000-9543-8.
Erard, Michael (January 3, 2004). "Just Like, Er, Words, Not, Um, Throwaways". The New York Times. Retrieved November 10, 2013.
Ing, John (September 26, 2019). "Pause Fillers for Speaking". Retrieved May 3, 2020.
Schuessler, Jennifer (November 9, 2013). "The Syllable That Everyone Understands". The New York Times. Retrieved November 9, 2013.