Really, that's the word. The study, by Mark Dingemanse, Francisco Torreira, and N.J. Enfield, claims that at least 31 languages from around the globe — from Mandarin and Dutch to Zapotec and Ghanaian Siwu — all include a "huh?"-like word that exists in relatively the same form and acts in the same way.
As linguistics papers tend to be, this one's long and highly technical (and an excellent weekend read for language nerds), but here's the gist.
"Huh?" can either act like a question word (similar to "what?") or as an interjection of "other-initiated repair," a signal that the listener has not fully heard or understood what the speaker has said; the authors claim that the latter signal is universal. Their study examines 10 languages in detail, comparing the pronunciation, intonation, and other characteristics of "huh?" In all but two of the languages "huh?" has a rising intonation, like you'd hear in uptalk. (In the other two languages, however, questions generally have a falling intonation.) Basically, it more or less sounds the same and acts the same in each language.
The authors also write that "huh?" is not generally considered a word in English, but rather a "non-lexical token" like "mmm" or "uh." They argue however, that "huh?" is a word because it is integrated into all the languages that they studied. Further, speakers of those languages must learn how to use the word in context, something that you don't need to do with instinctive utterances. ("Huh?" slightly varies and follows the rules of a given language; think of the word "ouch," which must be learned, compared to something like a baby's cry.) In the authors' words, "'huh?' is so common as to be practically universal, and yet calibrated to specific language systems such that it qualifies as a word."
The question, however, is how "huh?" could be both a legit word and universal. The authors cite several possibilities, but conclude that, most likely, the universality of "huh?" results from convergent cultural evolution. Essentially, all languages have conversation, and all have the possibility that a listener might not understand a speaker. Just as insects, birds, and bats all developed wings for the same reason, "huh?" developed because it addresses a universal need.
Make sense? If not, you can always just say, "huh?" (PLOS)