A lightbulb moment is when you suddenly understand something or get a good idea. It is the instant when a problem that was confusing before suddenly makes sense. This phrase comes from cartoon shows and movies, where a lightbulb appears above a character's head when they think of a clever idea. The image became a simple way to show the moment when someone's mind "lights up" with understanding.
I had a lightbulb moment in the shower and suddenly knew how to fix the problem with my bike.