A simple way to understand what consciousness is as told by Peter Russell in his book "From Science to God".

What is consciousness? The word is not easy to define partly because we use it to cover a variety of meanings. We might say an awake person has consciousness, whereas someone who is asleep does not. Or, someone could be awake, but so absorbed in their thoughts that they have little consciousness of the world around them. We speak of having a political, social, or ecological consciousness. And we may say that human beings have consciousness while other creatures do not, meaning that humans think and are self aware.

The way I shall be using the word consciousness is not reference to a particular state of consciousness, or a particular way of thinking, but to the faculty of consciousness - the capacity for inner ecperience, whatever the nature or degree of experience.

The faculty of consciousness can be likened to the light from a film projector. The projector shines light onto a screen, modifying the light so as to produce any one of an infinity of images. These images are like the perceptions, sensations, dreams, memories, thoughts and feelings that we experience - what I call the "forms  of consciousness". The light itself, without which no images would be possible, corresponds to the faculty of consciousness.

We know all the images on the screen are composed of this light, but we are not usually aware of the light itself; our attention is caught up in the images that appear and the stories they tell. In much the same way, we know we are conscious, but we are usually aware of the many different perceptions, thoughts and feelings that appear in the mind. We are seldom aware of the consciousness itself.