I don't believe in qualia as a real entity, but when people talk about them they're referring to a genuine phenomenon which you also experience: that your conscious understanding of the experience of perception is only the merest shadow of the perception itself. Seeing red doesn't mean seeing something with a little XML "red" tag attached, but something much more complicated that happens beyond your conscious introspection. You can imagine the state of having switched that "red" experience with the "green" experience, in all your memories as well as in current perception, and still instantly knowing that the switch had occurred. This phenomenon is not an illusion, just a blind spot of conscious knowledge which happens to confuse the hell out of naive philosophers.