The word "um" is apparently meaningless, yet it might have some meaning. Depending on the context, it might mean, "Don't interrupt me! I need to pause for a second, but I'm not finished talking yet." In response to a question, it might mean, "I need a few seconds to consider my answer. Don't assume I'm ignoring you or I didn't hear you just because I didn't answer immediately."

This was brought home to me one day when I actually heard someone SHOUT the word "um". It was in a factory, the two people were several feet apart, and they had to shout to be heard over the noise of the machines. One said, "LARRY! WHEN'S THIS JOB GOING TO BE DONE?" and Larry shouted back, "UM!...IN ABOUT AN HOUR!" I had to laugh.