Qualia are aspects of subjective conscious experience. The discussion of qualia tends to come up on LessWrong in two contexts: as an argument against reductionism (with a claim that qualia cannot be a mere matter of, uh, matter), and as a key factor in how seriously we should weight the suffering of animals. There is also a third context for Qualia arguments: as an argument against Whole Brain Emulation being you, or of emulations of minds not being conscious and instead P-zonbies.
There are several philosophical discussions of how to reduce qualia into functions of the brain (most famously by Daniel Dennett), but the discussion continues among philosophers.
Merge into https://www.lesswrong.com/tag/consciousness
Necroing here, and also marking it as a merge candidate into the Consciousness tag, since they are essentially the same thing.