Just like it sounds, radio-opaque means that the substance in question is opaque to radiation. Whether that radiation is high-energy like x-rays, or low-energy, like radio, is a matter of context and usage. What the term in typically used for is to mean "opaque to x-rays", however.
For instance, the big lead apron that the dentist wears when she shoots film of your teeth is radio-opaque. Thus the x-rays don't shoot through her and give her breast cancer by scrambling up her chromosomes like a boggle game.
Radio-opacity is very useful for smuggling shit through airport security. Depending on what you want to get through the machine, and the x-ray device you're trying to phreak, you want the "stealth" item to reside in the "shadow" cast behind the RO item. However, even if it's on the side facing the emitter, you're still ok.
Fancy 3-D CAT type xray machines blow this wide open, since they image all the way around the RO stuff, so you're busted, unless you're hiding the item in question inside a box fabricated out of radio-opaque material