I just read it off the site, man. Don't shoot the messenger.
It means that Superman doesn't have TRUE x-ray vision.
i.e here
Superman's eyes emit x-rays, which he uses to see with? That's not how x-rays work. They require a source that aims the x-rays toward the receiving end, whether it be eyes or photographic film. No, Superman's vision involves sensing energy fields that have hitherto been unidentified by human science. These energy fields surround and pervade all forms of matter, varying by density and vibratory rate, according to the density and composition of the object. In other words, Superman is seeing the subtle energy fields involved in the inter-transformation of energy into matter. His ability to distinguish those fields depends upon the "signal-to-noise ratio" between any object he is sensing and any intervening objects. Lead, being dense, has a field so dense that less-dense fields behind it are hard to distinguish.