The feeling of a separate "I". The ego prevents to see the Reality as it the Ego is occupied with desire, clinging, craving, aversion, anger, hatred and so forth. The Ego is an illusion however as it depends on desire it seems very real. The more one generates desire the ego grows and seems very real. If no desire, clinging or craving exists there is no Ego. The Ego will always try to make something important as it needs desire / craving to survive. Understand the non-importance of most things that the Ego tries to make you believe are important. The objects are excuses for the activity of the Ego. The Ego can only survive in time, worries about the past, desire towards the future. Hence Focus on The Now.