Mentioned sparsely throughout LISA: The Painful RPG, the White Flash was the apocalyptic event that led to the downfall of society in Olathe. For unknown reasons it supposedly lead to the death of nearly every human female, making Buddy the last known living woman.
Nern's dialogue seems to contain a lot of context about the effects of the White Flash. Nern described the event as a sudden happening, having been asleep only to be awoken by a bright white light. This effect is identical to that of the initial blast of a nuclear detonation: a massive, bright, white flash.
The White Flash has also had some effects on the day/night cycle of Olathe. Nern claims that days seem to last forever while night almost never arrives in Olathe.