A flash is climbing a route cleanly on the first attempt, but with the benefit of prior beta — the climber has seen someone else climb it, watched a video, or been told the sequence in advance. The body is fresh and the climb is unrehearsed, but the climber has knowledge of where the holds are and how to use them.
Flashing sits between onsight (no prior info) and redpoint (multiple attempts). It's harder than redpointing because the climber gets only one try without falling, but easier than onsighting because the beta removes the route-reading challenge. Most climbers flash 1–2 grades below their redpoint level and at or just below their onsight level.
Flash is also a standalone verb: "I flashed the V5." In bouldering competitions, flash attempts often score higher than redpoint attempts on the same problem. Watching a stronger climber send a problem before trying it changes the climb from a project to a flash candidate — beta dramatically reduces the difficulty of any moderate-grade route.