I'm a slow talker. If you were being kind, you might describe the pace of my pace as "pensive." If you were being accurate, you might describe it as "bumbling." As a result of my slowness, I say "uh," and especially "um" a lot. A whole lot. So much, in fact, that my son frequently demands out of frustration that I "stop saying um!" In the literature, such verbal fillers, and the related, "er," "I mean," "you know," and in children from the 80s and the Aughts, "like," are mercifully referred to as disfluencies, or even more mercifully as "performance additions." And over the last decade, psycholinguists (not a very merciful name) have found that they may actually play important roles in speech.
Traditionally, "uh" and "um" were thought to be involuntary products of a momentary difficulty in processing what one wants to say, or in deciding whether one actually wants to say it, and therefore are meaningless themselves. Alternatively, they were seen as merely a means of preventing people from interrupting during a pause in speaking. There's an obvious problem with this view, though. Why do we have more than one such marker of disfluency? In fact, English isn't the only language with more than one. Clark and Fox Tree1 found equivalents of both "uh" and "um" or similar fillers in multiple languages, including Japanese, Spanish, Norwiegan, Swedish, Dutch, French, German, and Hebrew. Distinct elements with no differences rarely survive in a language, much less several languages from different families, so there must be something to "uh" and "um."