1.
um; er; well
A filler used while the speaker searches for words or braces to say something awkward. Drawn out from その, it signals that the sentence is coming but not yet ready, and it holds the floor so the listener waits.
Usage
Sentence-initial, and often followed by a pause. It usually precedes bad news, a refusal, or a request, so it also works as a softener — starting straight into the point can sound blunt. Also written そのう.