They are fill words that show the speaker isn't sure of the topic. Toastmasters organization can help eliminate them (you obtain a point off for each "um" "uh" etc. that you use in your presentations.)

It's best when having to think when speaking (imagine that!) to pause and leave silence rather than an um.

It takes longer to listen to someone who uses fill words, and can become boring or annoying if constant. There are many preachers and motivational speakers I know who might not use um or uh but have their own catch phrases that provide a similar effect: "now listen", "anyone out there?", "if you know what I mean" etc.