X-rays are a form of electromagnetic radiation that is shorter in wavelength and higher in frequency (and thus energy) than visible light. They are able to pass through many things that are opaque to visible light, which has made them useful for medical imaging. Photographic material made to be sensitive to X-rays can capture an image from those rays after they pass through a human body, where they pass easily through the flesh while being partially blocked by bones, creating an image of the bone structure and other hard objects within the body. The "X" in the name originally referred to the rays being unknown.
Alas, the "X-Ray Specs" that were frequently advertised in comic books in the old days did not actually work; they were a (rather lame) illusion.