Simply put, the Hard Problem asks the following question: how can the machinery of the brain (the neurons and synapses) produce consciousness — the colours that we see, for example, or the sounds that we hear?
https://www.abc.net.au/news/2017-07-07/david-chalmers-and-the-puzzle-of-consciousness/8679884
“Consciousness baffles me, but not the Hard Problem. The Hard Problem arises only if one makes a metaphysical assumption, namely that the intrinsic nature of the world’s quantum fields – the essence of the physical – is non-experiential.”
— David Pearce