First we should define what a plane is: existence is viewed as a physical phenomenon by the rational western point of view; consciousness is just an attribute of some of the elements of existence.
from an esoteric point of view, the physical is just a part of existence, in fact the smallest part. We could use geometry for analogies, in geometry a plane is a space confined by some borders, and a plane has two dimensions, length and width, if you add a third dimension, depth, then you got an object instead of a plane.
Now, esoterically speaking, a plane is to existence, what a dimension is to geometry, a whole new level of existence, you can be a line: length, one dimension, a plane: length and width, two dimensions, or something like a cube or a cone or any object: three dimensions, width, length and depth. and all them are simply different forms of conceive projections in space, esoterically, a plane is away to project existence, a way to exist.
Plato said that he thought that the world of ideas was real, because things in that world can be perfect or eternal, he presented the analogy of a cave. if there’s some people on a cave, and thy are talking by a bonfire in the night, someone outside the cave can see the shadows projected by the people moving, and hear the voices, somehow distorted, but the sound and the shadows are not the people, the people are the people. For Plato the physical world was like the shadows and sounds perceived by the stranger, and the world of ideas the actual cave and its occupants.
That world of ideas IS the astral plane, and this physical world is a projection of the astral the bigger difference between both is the density of the matter, not in physics terms, but in existence terms, the physical is more dense and the astral more subtle, therefore more flexible freer, and vastly bigger than the physical plane, and you know the physical universe is quite quite big.
Here comes the catch, the heavens and hells of most cosmovisions are located in the astral plane, and if you arrive to heaven you think: thats it, this is as good as it gets right?, wrong; theres more.
As the physical world is a reflection of the astral plane, the astral plane is a projection of another plane called Causal or Mental or Supramental by some gnostic traditions.
Hindu think that existence is limited to an endless cycle of incarnations in this planes, don’t confuse body and birth with physical, you also have an astral body, and a causal body and you can take birth in those planes.
The mystic adept tries to escape that cycle, for there is the thing, the BIG thing, all the inferior creation: physical, astral and causal, is a reflection of the true creation, the spiritual creation, that level of existence without any kind of materiality, and that creation also has different planes.
The fun part is that all planes coexist in “the same space” or place, and that you can willingly travel across them.
Now the inevitable question, yeah all that accord to whom?… well its been said by mystics of all traditions in the terms of their own cultures, if you want biblio from eastern and western sources i can give you some titles, that includes, Hindu, budhists, Parsi, Christian and Muslim sources, but none of them care for religious boundaries, in fact they try to abolish them.
The most important thing is that all this can be empirically experimented, don’t dies on theory, magic is one of the many ways to do it, or a name that engulfs all the ways if you wish.