By Dr. Ken Broda-Bahm:
Back when I taught public speaking in college, I had a colleague who was so committed to exorcising her frequent use of "um," that she asked an associate to sit at the back of her classroom and hold up a sign with "um" written in foot-high letters every time she made that sound. That, of course, is the overreaction of a perfectionist. But most speakers -- particularly the less experienced ones -- are troubled from time to time by their tendency to add "um" or "uh" or "oh" to their speech. In practice, however, it is generally not worth the worry. Aside from being forgiven -- particularly for less experienced speakers -- these communication characteristics are shown by the research to be not just generally benign but actually helpful in some circumstances.
The "um," the "er," or the "ah" are referred to as a "filled pause," because that is how they function. Speech-wise, it is simply a pause to which you've added a vocalization. You make a sound essentially to hold the floor and to keep the communication channels open while you think of the next word or phrase. From an audience's perspective, much of the benefit comes from the pause: Listeners need a break and a chance to focus on what is coming next. But some of the benefits are independent of the pause, because they play a role in conveying and augmenting the speaker's own thought processes. That doesn't excuse a speech that is chock-full of disfluency, but when it comes to the normal and natural use of a few "um's," this post aims to reduce your guilt.