They assert that Mary understands the
physical phenomenon of red. That is, she understands photons and eye structure, and therefore knows that light of a particular wavelength will trigger these sensors in the eye and thereafter be interpreted as "red" by a brain. All the physical components necessary to produce and sense the color "red". But when Mary sees the apple for the first time, did she learn something more about "red"?
Also, it's a thought experiment. Some people will claim the answer to that question is no, she learned nothing. Others will claim that she did. It's that thing she learned beyond the physical that theoretically cannot be conveyed by science, or even possibly by language.