Papanca is the mind's habit of turning one plain perception into a pile of commentary. The classic image from the Buddhist texts: you feel something, then label it, then think about it, then build a whole world of judgment on top of the label, until the original, simple contact is buried under three imagined conversations and a mood. Buddhist teachers trace the pile back to three roots: craving, conceit, and views, the mind reaching for more, defending its rank, or arguing its opinions. It is not just "too much thinking." It is thinking that manufactures a self and a world out of a single raw moment. Notice it this morning at the first stray thought about an email or a comment someone made. Watch how fast it sprouts a second thought, then a third, then a whole scene. Before the scene finishes building itself, come back to the one plain thing you actually noticed.
the propensity of the [worldling's] imagination to erupt in an effusion of mental commentary that obscures the bare data of cognition
Bhikkhu Bodhi, note 229 to The Middle Length Discourses of the Buddha, as quoted in Andrew Olendzki, "What Is Papañca?" (Lion's Roar)