For instance, the light paths wiki says:
Shadow: the ray is used for (transparent) shadows
It then shows a diagram with the ray labeled “shadow” going (backwards) from a surface to a light source.
This doesn't make much sense to me. The text doesn't make sense because in cycles (as far as I know) a shadow is just an area with less light than the areas around it, which is not a technical value.
From the diagram, it looks like a shadow ray is a ray that is about to hit a light source (speaking from the actual backwards ray-tracing point of view). But I don't understand how that works either, how can cycles know what a ray is about to hit? Especially if that information could be used to change the direction of the ray.
So what is a shadow ray?