Usually reflective surfaces change the polarization of light in a predictable way, and some aquatic predators use that property to look for food. But fish skin is complex enough that reflected light isn’t clearly polarized, and thus the fish are less visible and less likely to end up as lunch.
Fish skin structure explains biological cloaking.
By the way, coverage of this story has includes ridiculous (“rubbish” as my English editor says) claims that the fish skin breaks the laws of physics. I can only assume that the writer was overenthusiastic and untutored in science. The article linked here claims “biological cloaking” which also seems like a bit of a stretch. What is really going on with the fish skin is that it is camouflaged in ways that we can’t see with human eyes.
The Science of Heroes 