Well, take plain old Mary. She knows, in her black and white room, all the physical facts about human color perception. E.G. when we humans see red our brain is in
state X. Now she sees what we tell her is a patch of brilliant red. She knows, for the first time in her life, that her brain is in state X.  That's because her brain is in X FOR the
first time. But she also learns
something more, what it's like for us humans to see red. She learns it by tokening the phenomenal property that us humans token when we see red,  and being introspectively acquainted with it.
This is, after all, a different fact. A blind person first seeing red, who knew nothing about neurology, would
learn the second fact but not the first.

By hypothesis, whatever it is that Mary Zombie learns when she sees the patch of brilliant red, it isn't the second fact.
That's what it is to be a zombie.