Wikipedia: Fillers are parts of speech which are not generally recognized as purposeful or containing formal meaning, usually expressed as pauses such as uh, like and er, but also extending to repairs (”He was wearing a black shirt and uh, blue pants”), and articulation problems such as stuttering.
…Some have hypothesized that the time of an “uh” or “um” is used for the planning of future words; other researchers have suggested that they are actually to be understood as full-fledged function words rather than accidents, indicating a delay of variable time in which the speaker wishes to pause without voluntarily yielding control of the dialogue.