I think in a most basic sense it just means "outside", the "other". I am aware that my "I" is not all that there is, that there is something outside of me, that the world is a "given", and thus a gift. I perceive that this "outside" is greater than my "I", that I belong to it. This to me is the first impetus towards religious worship and devotion, understood not as something commanded but as a spontaneous act of awe before the mystery of the greatness of this "other". Especially in the context of ancient religion in which people perhaps lived more "face to face" with that awe and with the world, without as many of the intermediaries of technology and artifice.
At the same time, I discover that "something" inside as well. Transcendence and Immanence are like poles that are intrinsically related. A purely transcendent God would make no sense, we could have no experience of such a thing whatsoever. A purely immanent concept of deity runs the risk of falling into some pure egoism, pure solipsism. I discover that the Whole which is entirely beyond me is nevertheless also present to me. Interior intimo meo, "my innermost self", Augustine said. "An invisible and subtle essence is the Spirit of the whole universe. That is Reality. That is Truth. That thou art." as the Upanishad says.
