This selection in Chalmers includes a paper in which Jackson presents the example of Mary that you've seen Block refer to and also the bulk of a short paper in which he replies to some objections. The term qualia in the title is the plural of the noun quale (kwayli in the standard barbaric pronunciation), which can simply mean 'quality' but, in this context, refers to the felt or phenomenal qualities that are "what it's like to ...."

Actually, Jackson considers three arguments for consciousness or the qualia that comprise it, and you should give each some attention. But it is the first of these, the "knowledge argument" associated with the example of Mary, that will be the focus of our attention. And it is objections to that argument that Jackson responds to in the second selection Chalmers adds as an addendum.

Jackson also gives the most extended account of issues concerning epiphenomenalism that you've seen so far, and we will spend a little time on that, too, so you might think whether you are sympathetic to this position (as Jackson is) or share Smart's doubts about "nomological danglers" (see Chalmers, p. 61).