That’s the part of you that exists eternally in the God Time. It is part of you and means that you always exist partially in the Hero Plane. It can be encountered by other heroquesters, it can cause you to perceive the otherworld even when you do not. It is a conduit between the God Time and the Mundane World, and is the source of a hero’s power, but it also is a source of disassociation from the Mundane World.
Do all mortals have a hero soul? No. Most have not awakened one.