David Chalmers, answered from the texts and cited to the page.
The hard problem of consciousness is the problem of explaining why and how physical processes give rise to phenomenal experience — to the qualitative subjective aspect of mental life, to there being something it is like to be a conscious organism.123 It is distinct from the easy problems of consciousness, which concern the cognitive and behavioural functions associated with consciousness: discrimination, integration of information, the reportability of mental states, the focus of attention, the difference between wakefulness and sleep.4
The easy problems are easy in a methodological sense: we have a standard paradigm for addressing them, namely identifying a neural or computational mechanism that performs the function. The hard problem resists that paradigm because explaining any cognitive function leaves a further question open: why is the performance of that function accompanied by experience?
Why doesn't all this cognitive processing go on "in the dark"? The hard problem is not a denial that consciousness exists, nor a claim that it cannot be studied scientifically, nor a brief for mysterianism; it is a precise claim about the form an adequate theory of consciousness must take, and the form it cannot take.5
The Meta-Problem of Consciousness David J. Chalmers The meta-problem of consciousness is (to a first approximation) the problem of explaining why we think that there is a problem of consciousness. Just as metacognition is cognition about cognition, and a metatheory is a theory about theories, the metaproblem is a problem about a problem.The Meta-Problem of Consciousness, pp. 1–2
Th ere is something it is like to see a vivid green, to feel a sharp pain, to visualize the Eiff el tower, to feel a deep regret, and to think that one is late. Each of these states has a phenomenal character; with phenomenal properties (or qualia) that characterize what it is like to be in the state.Consciousness and Its Place in Nature, p. 105
This is why I call these problems the easy problems. Of course, 'easy' is a relative term. Getting the details right will probably take a century or two of difficult empirical work. Still, there is every reason to believe that the methods of cognitive science and neuroscience will succeed. The really hard problem of consciousness is the problem of experience.Facing Up to the Problem of Consciousness, pp. 2–3
The hard problems are those that seem to resist those methods. The easy problems of consciousness include those of explaining the following phenomena: the ability to discriminate, categorize, and react to environmental stimuli; the integration of information by a cognitive system; the reportability of mental states; the ability of a system to access its own internal states; the focus of attention; the deliberate…Facing Up to the Problem of Consciousness, p. 2
We can say the same 2. It is sometimes suggested that arguments for the explanatory gap turn on an outmoded "deductive-nomological" model of explanation. So it is worth noting that this chapter invokes a model of explanation in terms of functions and mechanisms.Facing Up to the Problem of Consciousness, p. 8
"Facing Up to the Problem of Consciousness" (JCS 1995) and its restatement in The Conscious Mind (1996) Chapter 1 placed the hard/easy distinction at the centre of the field. The 1996 JCS symposium produced sustained critique — Dennett, Churchland, Hardcastle, O'Hara & Scutt, Price, T.W. Clark — collected in Shear's Explaining Consciousness (Bradford/MIT 1999). The deflationary critique (most recently in Keith Frankish's illusionism 2016) denies that phenomenal consciousness as Chalmers describes it is real. The Meta-Problem of Consciousness (2018) reframes the dialectic: explaining why we make problem reports about consciousness is itself an easy problem, and the question is whether its solution dissolves or merely constrains the hard problem.