The term filler also has a separate use in the syntactic description of wh-movement constructions (see below).
Every conversation involves turn-taking, which means that whenever someone wants to speak and hears a pause, they do so.
Pauses are commonly used to indicate that someone's turn has ended, which can create confusion when someone has not finished a thought but has paused to form a thought; in order to prevent this confusion, they will use a filler word such as um, er, or uh.
[4] Filler words generally contain little to no lexical content, but instead provide clues to the listener about how they should interpret what the speaker has said.
It refers to the pre-posed element that fills in the "gap" in a wh-movement construction.