Um’s and uh’s indicate that the speaker is “in trouble.” The primary view on the purpose of filler is that it is either an involuntary symptom or a purposeful signal (here linguists do not agree) that the speaker gives to indicate to his listeners that he is “in trouble” – he needs a moment to plan what to say next or to hunt for something in his memory. It tells the audience that there is about to be a delay. “Uh’s” signal a shorter delay, while “um’s” tell the audience the delay will be longer.
Basically, um’s and uh’s happen when you’re trying to think and speak at the same time. This is why they occur more frequently during transitions to a new topic or at the beginning of a sentence rather than at the end or in the middle of one; your brain is idling at the juncture between planning and executing what to say next.
Toastmasters is a great options for practicing your presentation skills (and working to eliminate these fillers).