The best sound bite I've heard for this one is that consciousness is a story the brain tells about itself. The idea is that what we experience as consciousness is a feedback loop of the brain producing and receiving its own stimulus. In addition to processing actual sensory input, out brains are also adept at creating signals which it can then process as if those signals were sensory input. What you eventually get is a constant hum of brain activity in which the brain is describing its own state