There's a question that's puzzled many a traveller on the path!
First we have to have a definition of ego. For me, ego represents my internal dialogue, my brain programs and the like. As you can see it's still a bit vague, but simply it's anything that gets in the way of direct experience of universe. You know those moments of pure peace and contentment when time seems to stand still and all your worries are seen for the illusions they are, when you feel part of nature and you require neither questions nor answers? Well that's when ego is out of the way :-)