um

A filler word or hesitation noise.

These are used to indicate 'I'm not done talking yet' when used in a sentence. It informs the listener that now is not the time they should jump in with their views on the subject -- they should wait until you finish this thought.

As most speeches, monologues, and dialogs performed in front of an audience are pre-rehearsed, we tend to see ums, ahs, and ers as failings in speakers.

They are also used (Um especially, in America) to let the other person know that you don't quite understand what they're saying. In this case it would be used after they have finished their sentence or thought. With luck a well placed 'um' will goad them into elaborating.

I was once told by a very nice flight attendant on Air Canada "You've got an UM sitting next to you," upon which I turned to look at the empty seat to my left, and then back at her, quizzically.
"Um?" I said.
"Yes," she said, "an Unaccompanied Minor. She's a twelve-year-old and English, like you." We were flying from London, so it was hardly surprising. "Oh, here she comes" said the attendant, and after some initial fussing over the small, silent girl, who had a mini-sandwich board with UM in big red letters around her neck, she left to go and be nice to some other people. The girl turned out to be going off alone to visit her grandmother in Vancouver, as she had done for the previous three years.

See also Canadians can't help being nice