Is it like another deminsion or something? 
Well, for those who believe in it, they aim for AP, or Astral Projection. It’s supposedly a higher plane of existence with higher beings (if I recall correctly). Here’s the definition in the chatroom:
[22:42:41] AP == Astral Projection - the practice to project the consciousness to a realm different from the physical body.
Some say LDs, OBEs, and AP are all the same thing, but if you don’t think so, I’ll define them this way. An LD is a dream you’re aware in, an OBE is when you’re essentially in the “real world” and out of your body (often in a process much like WILD or DEILD), and an AP is when you’re in a different plane or higher plane (also with a process much like WILD/DEILD).
Yep, Astral Projection is the art of inducing an out of body experience. Everyone does it every night, like dreaming.
If you do a bit of research on Google, nobody really comes to a mutual agreement to what the difference and similarity to the Astral Realm and the Dream Realm really is. The video I watched the other day, he said that with no shadow of doubt, all Lucid Dreams are in-fact Astral Projections. Yet, long ago on another Lucid Dreaming centred forum, one guy said that Astral Projection is just mere Lucid Dreaming were the Physical Realm just materialises itself into your subconscious.
Now if you want my take on this, I don’t really care, if you have the mindset that they’re pretty much the same thing, you’ll get somewhere, and that somewhere is where you begin to get on your feet as to what things are.
ZRVera gave a pretty good description, so I’m pretty much just continuing and agreeing.