New York Times:

Communications experts describe “um,” “aah,” “you know” and similar expressions as discourse markers, interjections or verbal pauses.

They often occur when we are trying to think of the next thing we are going to say, Susan Mackey-Kallis, an associate professor at Villanova University who teaches public speaking, said in an email.

When stakes are high or we are nervous — in a job or media interview, or during a speech, presentation or conference call — we tend not to breathe as much and we talk faster, so our words get ahead of our thoughts

With the explosion of podcasting comes an equal explosion of people not trained in public speaking. The funny thing is the fix is easy but hard to do. I used to have a really bad stutter until a family friend explained why I did it and gave me a couple of simple exercises. One was, “Don’t say anything until you know what you’re going to say” and the other was, “just stop talking and think about what to say next”. Now, 99% of the time, I no longer stutter.