The normal experience of looking at an object is to feel as though there is nothing at all between your conscious and the object. It feels as though you are directly observing the actual object itself - but this is not the case.
For a start, there are the obvious intermediaries - the lens, the retina, the optic nerve, and so on - but the real magic of the experience of consciousness is something that exists above all that. We don’t simply become aware of the location and properties of the object, as we might assume a computer system would, we have an conscious experience of it.
Our awareness - by which I mean our consciousness at it’s most impersonal and fundamental level - is like a passive observer, with things appearing in a field before it. Throughout life, this awareness is observing a constructed inner world; a colourful, spatial, audible, tactile, emotional world. It is our minds which render a blade of glass to look green, or the sky to look blue. The brain generates the experience of colour, it doesn’t exist in the same sense in the outside world.
This idea extends beyond vision to all of human experience. We do not experience the world directly, we are an awareness (within a mind, within a body) which observes that which the mind generates. Your reality is the understanding and experience you have of the world, but those are internal phenomena. The illusion of consciousness is to seamlessly aggregate these experiences, and to make the internal reality feel external.