I'll try my best to answer this from my experience.
The astral plane is a collective term used for fourth (and fifth?) dimensional planes of existence, other than our physical plane, that vibrate at different rates. The astral plane is made up of Akasha.
We have the *MANY*, *MANY* other planes and dimensions that have their own worlds and systems. These range and vary in vibrations.
Low level dimensions/planes have less light in them and are known for holding negative and low-level beings in them. It looks like a bit after sunset. The lower the plane, the more 'night; it becomes.
High level planes and dimensions have more light in them, beautiful landscapes, and more friendly the beings become. It always looks like daylight here.
A being or entity from one plane of lower vibration cannot cross or exist in a plane of higher vibration. This is due to their differences in their vibratory rate. BUT a being from a parallel plane can cross over to a similar plane. This very rarely occurs and the being generally doesn't exist in it's new field for too long.
One of the strangest things i've seen on a lower plane is a little boy that "fell in" & got stuck in it somehow. The place he was in looked a lot like a hot, stinking, primordial swamp filled with modern day garbage (old car tires, household junk etc.). There were other beings, some quite extraordinarily technologically advance that were picking on and taking advantage of him for their own purpose. He got older, became a mature man, and his future self was trying to find away to stop his "boy self" from falling in. Both versions of himself existed at the same time. The passage of time seemed irrelevant here.
Our minds can tune in and manipulates Akasha to some extent, This how communication such as telepathy and psychic communication is done. Akasha deals with space and time.
I believe the Gods use the fifth dimension in their own affairs. Explaining this would be very complicated as it's a it goes beyond my understanding of time and existence. It is best experienced to be understood.
Hail Satan.