It is the fundamental theorum of why the universe makes sense.
Light is neither a particle nor a wave. It's something that we don't have a word for and that doesn't exist in a way that we can sense directly. But this unnamed thing happens to act in a way similar to a wave in some situations and like a particle in others. A cylinder will roll like a sphere in one direction but not roll like a cube in the other. That doesn't make it a sphere and a cube at the same time. It makes it something different.