Humans visit Dream every time they go to sleep: every human has a shadow-self within Dream which awakens just as the human's real self falls asleep. This is part of the fallout from the First War, the faintest influence of the Unreal on men's minds. Once it was apparent that this was happening, the Gods established a green and peaceful land on the border of Dream, where humans tend to find their shadow-selves when they go to sleep. About the green and peaceful land was dug a deep trench, extending downwards for miles, at the bottom of which lurked the nightmares of men - frightening experiences which would do no lasting harm but would encourage stray humans to retreat to the lands they know.
Sometimes, through training or chance, humans can leave the green land of pleasant slumber and the pit of nightmare and wander the wider landscape of Dream. Individuals who achieve this by training are called dreamwalkers, and dreamwalking is a potent form of magic.
What is so strange about Dream that the Unreal like it so and the dreamwalkers find it so valuable? It is a place of flux, of instability, of reality itself gone feral. Magical acts become much easier to achieve in Dream. Folk can shapeshift entirely by accident. Mad native creatures scream obscure wisdom from the top of mountains. The truth of one's life is laid bare in a short card game played with a rhinoceros, a dragon and a small child (the child wins). Dream is where Creation's seams show the most: if Creation is in trouble, Dream is where the first effects of that will be seen.
The only thing in Dream which is constant is the threat of the Unreal. War-bands of folk pledged to the Unreal and tainted with their touch roam the land. Their fortress tempts the desperate and lures the unwary, for they can grant many things which even the Gods dare not grant: freedom from death, the absolution of Oaths, the breaking of the rules of the world...
During the Iron Rule a second Wall of Dream was built by the Gods of the Iron Laws, a pitiless Iron barrier which imprisoned yet more lands in Dream. Whilst this was necessary - the divine custodians of the Wall of Dream had been imprisoned by the Iron Rule, so the Unreal could slip out - the Iron Rule also used it as an excuse to put nations it did not like (including the Kingdom of Grace) within the bounds of Dream.
Now that the Iron Rule is over, the Iron Wall is fallen and the First Wall of Dream is once again a potent barrier.
System-wise: Visiting Dream requires getting past the Wall of Dream. This is a Legendary action for most gods - not only is the Wall a mighty defence, but finding it is difficult, for the paths to the edge of Dream are hidden. Rumour has it that the members of the Court of Barred Doors have their own way of entering Dream.