I find David Chalmers's explanation of what is meant by "qualia" and "subjective experience" and "something its like to be" the easiest to understand. For example, read the first chapter of <i>The Conscious Mind</i>.
The Knowledge Argument above refers to the fictional story of Mary the color scientist. She was raised in a black and white environment and never saw color. But she read textbooks on color theory (printed in black and white, of course.) The question is, when she finally experiences the color blue, how is that different from the previous knowledge she had about what the color blue would be like? That different extra aspect to the actual experience is what we refer to as qualia, and how such an experience can be caused by physical processes is (in Chalmers's terminology that has now been widely adopted) the Hard Problem.