Neuroscience has made enormous progress mapping the physical processes behind perception: light of a certain wavelength hits the retina, specific photoreceptors respond, and a cascade of neural activity travels through the visual cortex, ultimately producing the experience of seeing red. Every step of that causal chain can, in principle, be described in purely physical terms. What none of it explains is why any of it is accompanied by subjective experience at all — why there's something it's like to see red, rather than the entire process happening in total experiential darkness, the way it presumably does inside a camera or a thermostat.
Two kinds of fact that don't obviously reduce to each other
The mind-body problem asks how mental states — sensations, thoughts, subjective experiences — relate to physical states of the brain. Physicalism holds that mental states simply are physical states, fully explicable in principle by a complete physical description of the brain, with nothing extra required. Dualism holds that mental states involve something genuinely non-physical, distinct from and not reducible to brain activity. Philosopher David Chalmers gave the puzzle a now-standard name: the "hard problem" of consciousness — distinguishing the "easy" problems (explaining how the brain processes information, controls behaviour, and integrates sensory input, all of which are difficult but appear to be solvable through ordinary science) from the genuinely hard problem of explaining why any of that physical processing is accompanied by subjective experience in the first place.
Why "just more neuroscience" doesn't obviously close the gap
A famous thought experiment sharpens the difficulty: imagine a scientist named Mary who has lived her entire life in a black-and-white room and has learned every physical fact there is to know about colour vision — every relevant neuron, every wavelength, every neural pathway — without ever having seen colour herself. If she then steps outside and sees red for the first time, does she learn something new? Many people's strong intuition is yes — she learns what red actually looks like, a fact that seems to go beyond anything contained in her complete physical knowledge. If that intuition is right, it suggests subjective experience isn't simply identical to, or fully captured by, physical facts about the brain — which is exactly the gap the mind-body problem is about.
What we're still unsure about
This is one of the most genuinely unresolved problems in philosophy of mind, and it isn't for lack of serious effort — physicalists have proposed numerous ways to explain away the apparent gap (arguing Mary gains only a new ability or a new way of representing information she already had, not new factual knowledge), while dualists and other critics of physicalism argue no purely physical description will ever fully capture subjective experience. Neuroscience continues to make real progress correlating specific brain states with specific reported experiences, but correlating brain activity with experience isn't the same as explaining why that correlation produces experience at all rather than nothing — and no consensus exists among philosophers or scientists on whether that explanatory gap will eventually close, or whether it marks a genuine, permanent limit on what physical science alone can explain.
This sits inside The Mind-Body Problem, one of seven topics in Metaphysics, one of five domains in Philosophy, one of seventeen subjects the app can quiz you on.