Wilhelm Röntgen discovered that X-rays were capable of passing through solid matter, albeit to varying degrees depending on the density of the material. This was the groundbreaking aspect of his discovery.

Here's a breakdown:

* X-rays are electromagnetic radiation, much like light, but with higher energy.

* This high energy allows them to penetrate through many materials that are opaque to visible light.

* Denser materials, like bone, absorb more X-rays than less dense materials like skin and muscle. This difference in absorption is what allows us to create images of the internal structures of the body using X-rays.

Röntgen's discovery revolutionized medicine and other fields because it provided a way to "see" through objects and examine their internal structure, which was previously impossible.