Also known as "a moment of clarity," an upsight refers to when avout, or scholars of any kind who devote their lives to learning (using their brains a lot), experience a moment where some problem, which had them stumped, would suddenly become clear and understood.
During Erasmas and Orolo's several day long Dialog at Orithena it is made clear that the brain creates information (i.e. the consciousness) through quantum events. Furthermore, it is understood that the Hylean Flow, or crosstalk between cosmi, is also quantum in nature and can even be influenced (controlled even) by a very well trained brain, like fraa Jad. Thus, we can conclude that an upsight is likely information from "Up Wick" entering the brains of avout, or even sent directly to someones mind on purpose. This happened at least twice in the course of the story: first, when the idea to uncover Jules Verne Durand was given to Erasmas. Second, when the various Narratives in which mass destruction is brought down on the Daban Urnud when Cell 317 utterly fails, such as when Erasmas is present in a Narrative in which Jad activates the Everything Killers. Orolo further imparts to Erasmas that, like a quantum machine, the multiple waves of energy in your brain which are your thoughts, collapse down into a single wave, thus giving you the upsight.