Very basically put, the Astral Plane is the dream dimension, but at the same time it is not just that. It would be more accurate to say the dream dimension is part of the Astral. From another perspective, the Astral is like a huge mind net, catching and holding all thoughts, memories, fantasies, and dreams of every living thing in the world (and all other worlds for that matter.) In short, its contents are created by the collective consciousness of the universe, and you could say 'exploring the Astral' is almost another way of saying we are 'exploring the inner mind of the universe'.

"All of the dreams people have form a mass dream framework. Dreams exist at other levels, and physically of course affect the body state. In such ways, the world's actions are worked out in mass dream communications that are at the same time public and private."

- Seth, Dreams, ?Evolution? and Value Fulfillment p.252

That gives us some idea of what is meant by the 'Astral Plane', but again this is just the beginning of an answer. In answering this question more fully, it is going to be very helpful to first quote Seth on his answer to the question "what is a plane?":

"A plane is not necessarily a planet. A plane may be one planet, but a plane may also exist where no planet is. One planet may have several planes. Planes may also involve various aspects of time. Planes can and do intermix without the knowledge of inhabitants. A plane may be a time... or only one iota of vitality that exists by itself. A plane may cease to be. A plane is formed for entities as patterns for fulfillment along various lines. It is a climate conducive to the development of unique and particular capabilities and achievements... An isolation of elements."

-Seth, Seth, Dreams and Projections in Consciousness p.103

Oh, that clears it up then... I think... Well, let's say, when we talk about the Astral Plane, we are really referring to the combination of all the planes that exist on an Astral vibratory level, and this encompasses an infinite variation of 'realities', 'realms', 'sub-planes' or 'rings'. It may be easier to think of these as 'real dreams'. We can take note however that on all Astral levels, consciousness is clothed in some form or other, whereas on 'higher' planes this is not necessarily the case.

So is there an exact location to the Astral Plane? No, not really, since all levels and vibrations of reality exist and penetrate all at once. We may as well ask if there is an exact location to the entire physical universe...? Doesn't make much sense, does it? In relation to what? Instead, the Physical, Etheric, Astral, Mental and higher planes all 'interpenetrate'. It is more accurate to say that we 'tune in' and 'tune out' of the Astral with our consciousness, it is a 'within-ness' to things, but it is always 'there'.

To follow this analogy, we can consider that it's a bit like radio or TV stations. All the frequencies that these are broadcast on exist at once in the atmosphere, but we only perceive one at a time depending on what we are 'tuned in' to. Only in the case of the Astral, it is our consciousness that is doing the tuning. So the whole range of frequencies we call 'Astral', not 'radio' or 'TV', and the different 'stations' we call 'sub-planes' or 'rings' or 'levels of reality'.

Or we can use a travel analogy. You take it on faith all the countries of the world exist, even though you've not been to them, and you can travel to any one of them. In a similar way all the Astral Realms exist at once. By going to another country, your focus - you - is there, for however long. Astral Travel is similar, the difference being of course that it is a non-physical 'place' you are focussing in.

But we may well ask, does the Astral (and other dimensions for that matter) exist as consciousness within us or as worlds outside of us? And to make it all nicely trickier, the answer is both. Consider, if you and a friend both memorized the same poem you found in the library, would the poem exist within you, within him or outside of you both? As with many spiritual questions, all answers are correct, but either in isolation is technically 'false' because incomplete. It is the same for the Astral Plane. It's frequencies exist both outside of you and within you. You might even say it's like an 'inner world that you are within'.

To turn the question around, inside and outside are relative to your perspective, just as up and down are too. You can equally say that you experience physicality when you are 'inside' your physical body as you can say the physical is the experience 'outside' of the Astral and other dimensions. And vice versa. Since it is a matter of perspective, all answers are 'true', relative to where you are. Well, so much for words! Hopefully have some kind of picture now of what is meant by the Astral.