Basically it's the heart of all worlds, but if someone were to open it and keep it opened then it would cause a reset of the current worlds as it floods everything in light. Like the original one, artificial ones are also always surrounded by darkness (like the door to darkness in the first game) and so even trying to get to them could potentially flood all worlds in darkness. Only the x-blade can summon, open, or close the true kingdom hearts. Oh, and part of why the true, original world split from Scala Ad Caelum in the age of fairy tales is because after the keyblade war, the worlds themselves splintered off in order to prevent kingdom hearts from being accessed again. Which is to say the worlds do have a will of their own.
The entire goal for Xehanort was to use it to reset the world and create one of perfect equilibrium, and he would act as ruler and prevent darkness from overwhelming things and causing a keyblade war like in the current world.
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tfw no big tiddy goth vampire gf who lactates blood - viewmaster_pi