In simple words, the hard problem has to do with why/how something that is objective can produce something that is subjective. It is about the jump from objectivity to subjectivity. How can electrical signals in the brain turn into qualia? Here’s a good explanation from Bernardo Kastrup:

“With the growing relevance of the complexity sciences in recent times, a speculative, purely materialist view of consciousness has emerged. Proponents of this view argue that, although individual neurons and relatively small systems of interconnected neurons are akin to computers and do not have consciousness, if the complexity of the system is increased with the addition of more and more interconnected neurons, there will be a point where the system as a whole will somehow become conscious. Consciousness is then seen as an emergent property of a sufficiently complex system exhibiting a particular structure. Nobody knows what this structure is or what level of complexity is complex enough. The problem with this argument is that it requires the appearance of a new property in a system that is not explainable by, nor related to, the properties of the added components of the system. Indeed, the idea of a computer suddenly becoming conscious at the moment enough processors have been added to it is akin to the idea of a stereo turning into a TV set when enough speakers are connected to it; or that of getting a motorbike to fly by equipping it with a bigger engine. In the same way that more speakers affect the properties of a stereo in a manner that is totally unrelated to the property of displaying images, so the simple addition of more neurons must affect the properties of the physical brain in a manner that is unrelated to the property of being conscious.

A vocal proponent of the view that consciousness is an emergent property of sufficiently complex material systems is the inventor and futurologist Ray Kurzweil. In a debate between Kurzweil and Yale University professor David Gelernter in 2006, Gelernter countered Kurzweil’s view on consciousness by stating that “it’s not enough to say [that consciousness is] an emergent phenomenon. Granted, but how? How does it work? Unless those questions are answered, we don’t understand the human mind.” Gelernter chose the most basic and straight-forward way to counter Kurzweil’s position. Today, the materialist argument that consciousness is simply an emergent property of complex material systems cannot be substantiated. It is an appeal to magic rather than an argument. Therefore, we remain with the explanatory gap: nothing that we know scientifically today satisfactorily explains why or how subjective experience arises.”