The sense of awareness, so much so that you are able to change your internal state, not only that, but the world model you have about the world also includes a version of you which is interacting with that world model, and making active changes in that information base and using dynamic reference frames to make sense of the environment.

By no means is this complete, but this is in short what I think.

Check out Swami sarvarpriyananda and david chalmers to understand more about consciousness. One will explain through science and the other with spiritual science/lived experience.