Every language has its own version of um. French has euh, Korean eum, Finnish öö, Russian eh; even sign languages have signs for um. The fact that most languages have some kind of um suggests that it serves a natural and important language function.
So what is this important language function? Why do people say um? Not because they are nervous. Scholarly studies of the word reveal that the use of um does not correlate with anxiousness or any particular personality traits. Rather, um is used to signal an upcoming pause—usually uh for a short pause and um for a longer pause. The pause may be needed in order to find the right word, remember something temporarily forgotten, or repair a mistake. Um holds the floor for us while we do our mental work. It buys some time for thinking."
Read the full text here: http://mentalfloss.com/article/50173/when-and-why-did-people-start-saying-um-when-they-talk#ixzz2R13HfDuP
—brought to you by mental_floss!