Look at something red. Now try to say what the redness itself is, without naming other red things, without mentioning wavelengths, and without pointing. The difficulty you run into is the whole problem of qualia — and it is why consciousness resists the explanatory strategies that worked everywhere else in science.
The word names the qualitative feel of experience: the ache of a headache, the smell of rain on hot pavement, the specific blue of a particular sky. Philosophers reach for the phrase "what it is like" to point at them. There is something it is like to taste wine and nothing at all, presumably, that it is like to be a stone. Qualia are the difference.
What makes them philosophically explosive is a gap. Suppose neuroscience gave a perfect account of colour vision — every photoreceptor, every neural pathway, every downstream behaviour. That account would explain how you discriminate red from green, why you stop at traffic lights, how the word "red" reaches your tongue. It would seem to leave one thing untouched: why any of that processing is accompanied by an experience at all, rather than proceeding in the dark. This is the explanatory gap, and it is the engine of the hard problem of consciousness.
Two famous thought experiments sharpen it. Mary the colour scientist knows every physical fact about colour vision but has lived in a black-and-white room; when she steps outside, does she learn something new? If yes, physical facts did not exhaust the facts. The philosophical zombie is a being physically identical to you but with no inner experience — if such a being is even coherently conceivable, then experience is not simply identical with physical structure.
Not everyone accepts the concept. Some philosophers argue qualia are a philosophers' invention, an artefact of taking introspection at face value, and that a mature science of the mind will dissolve rather than solve the puzzle. Others hold that qualia are real and irreducible, pointing toward something incomplete in the physical picture. The honest report is that this is unresolved — which is precisely why it remains one of the most alive questions in philosophy.