Consciousness is the hard problem in philosophy of mind — David Chalmers's term for what makes it hard. The easy problems (how do brains process information, integrate signals, generate behavior) are scientifically tractable. The hard problem is why any of that information processing is accompanied by subjective experience — why there's "something it's like" to see red, taste coffee, feel pain.
You could imagine a being physically identical to you that does everything you do — including writing this sentence — without any inner experience. Philosophers call such a hypothetical creature a "philosophical zombie." If zombies are even conceivable, then consciousness isn't reducible to physical function. If they're not conceivable, the hard problem dissolves.
Theories proliferate. Eliminativists say consciousness as we conceive it doesn't exist — there's just the brain doing brain things. Functionalists say consciousness is what certain information processing IS. Panpsychists say consciousness is a fundamental feature of reality, more like mass than like digestion. Integrated Information Theory tries to quantify it. None has solved the problem to the others' satisfaction.
What's at stake: ethics (do shrimp suffer? do LLMs?), AI (could a machine be conscious?), and the basic question of where you and your inner life fit in physical reality.