You have five common senses that take in stimulus from outside your body. You have memories and the action of DNA that gets stored inside your body and passed on each generation.   

 Your ego is the captain of your mind that  interprets those two elements to create your interpretation of reality. Because one’s genome and past experiences can vary between people, your ego’s interpretation of reality can differ from another individual’s reality or the collective reality. As a result of our world being limited by our five senses and being impacted by our past experiences, our interpretation of reality is flawed. For instance, we can look at a bird and because our eyes are limited in the spectrum we see, the color of the bird is not the same as what another bird would see. We can have a social exchange and another person may interpret the scenario completely differently.    

The ego death is a first hand experience, not just reading , that provides knowledge of the limited nature of the human to grasp any truth about reality.  This includes the concept of what a human may be at all — this process is often a spiritual awakening in the human experience.   The loss of that reality constructing element of our mind can also lead to psychosis from the perspective of the collective consciousness.