It is a world of shared unconsciousness. A sort of hive un-mind shared by all living things, by pony and griffon, by hydra and dragon alike. Endless space without space, time without duration. A thought-world of appearance without substance.
You arrive here simply through being unaware. When the body gets too tired to sustain wakeful awareness, the mind becomes untethered from physical stimuli and flows here to allow the body to rest. The stream of unconsciousness flows along with its own weight. The dreamstream carries you along as it pleases as long as you remain unaware.
If you do become aware while dreaming, it usually results in you waking up; you realize the dream doesn’t exist, and so it ceases to exist. I suspect this to be a self-defence mechanism of sorts for the dreamworld. Since it is all thought, a thinking being, that is, a thought-creating entity, could shape it at will. The dreamworld protects itself by ejecting thinking entities that are aware and thus out of its control.
It takes special skill to trick the dreamworld and to dream lucidly. With certain tricks of the mind it is possible to cultivate a consciousness withing the unconscious. Something only you have control over, something solid that the fluid dreamstream can’t sweep away. Like a rock in a river you can hold on to to stop the current from taking you.
You can then hop on top of that rock, and if the footing is solid enough you can reach down into the stream with a hoof to alter the course of the flow, at least partially. Reach too far, and you fall back in. Be careful enough about it, and you can mess with people’s dreams. That’s how dream magic works. Kind of. I’m not good with metaphors.