Certain things are reducible to their substrate if the "thing" in its original manifestation is just an aspect of that substrate. So, if you touch a surface and feel that it is hot, you can pretty much explain the "what" by the "how," by explaining how "heat" is actually stored/conducted/constituted in materials, and how our body perceives this by means of different nerve endings which register certain things as hot. Or you could look at a huge arc of lightning, or simply the presence of an electric current at one location on a circuit and at another location, and explain that by means of a smaller-scale explanation of how electrons are working and so on.
But consciousness genuinely is harder to explain. You can SUPPOSE that it is somehow a "function" or an "aspect" of the matter generating it, sure. But that's a bit like saying: "whatever dark energy and dark matter are, they are just functions of baryonic matter. Just because we don't know how precisely that comes about, doesn't mean it isn't happening!" Well yeah, sure, possibly. But that still isn't the same as the first sort of explanations I just described, which work precisely because we HAVE models and mental imagery for all the variables involved, and we can follow the chain of deduction and genuinely *see* how a visible arc of lightning is an "aspect" of many smaller interactions. At some point in the chain of deduction you reach a leap of faith, a black box, a moment where you have to go, "and then somethingsomethingsomething happens here, and boom! you have consciousness." With the electron example, at every link in the chain of deduction you are going "uh huh," all the way to the end product.
No one has yet explained how matter jiggling together in just the right way creates the what-it-isness of your subjective awareness. Which isn't to say this will never happen, only that is hasn't been, just as much as dark matter/energy hasn't been proven to be a function of baryonic matter, though it's possible.
