Conventional materials interact with electromagnetic radiation like light or radio waves based on the properties of the material. We’re used to how glass bends light or how gold reflects light and so on, and in our everyday encounters with these objects we know what to expect because there’s nothing special about these materials usually.
Unless some crazed engineer replaced them with metamaterials, then things can get pretty unnatural. Take that gold for example. You’re used to seeing it all shiny and yellowish and even on the nanoscale that’s still true for gold. Unless an engineer were to alter the surface of the gold, making nanoscale structures that changed how the light behaves. Then the gold could be green or red instead of the usual yellowish tint.
Nothing about the gold’s chemical properties have changed, it’s still good old Au, atomic number 79. But the structures on its surface can change how we see it. These special structures take gold from a conventional material to a metamaterial.
You might be more familiar with the concept than you realize. One Stanford engineer likens metamaterials to TV antennae of old. To adjust the image quality you would wiggle the antennae around until the geometry interacted with the radio waves better. But the waves carrying TV broadcasts back in the day were centimeters to meters long, so the antennas that interacted with them were relatively big. If you want your material to interact with electromagnetic waves that are microns to nanometers, then the shapes are going to have to be just a fraction of those wavelengths.