The Dreamlands expansion contains two separate maps (smaller than 40x20, but larger than half-size - together the two maps cover more area than a 40x20 map). One map is the Surface World, while the other is the Underworld. Certain Areas on the map are Portals between the two maps, and when you are at one of these sites, you can travel back and forth. For instance, the Tower of Koth (Underworld) and Sarkomand (Surface) are one linked pair.
"Why would I want to travel around?" you ask. It's simple - both maps have certain Areas that contain what we are currently calling Citadels (for example: Kadath, or the Vaults of Zin). If you control all of the Citadels on a map, then you win immediately! This gives players a whole new way to win the game! You can be far behind on the Doom track, but pull off a victory by your effectiveness in the Dreamlands.
It's not as easy as it sounds either. The game comes with 6 plastic Zoogs, and a Bhole. Both of these species are controlled by game rules - not the players. The Zoogs scuttle across the Surface World, interfering with your cultists and your Gates, as a pesky annoyance. The Bhole, on the other hand, is a fearsome predator who periodically rises from the depths of the Underworld and can demolish entire armies. Both of these make it more challenging to get a "Citadel victory" which of course improves gameplay further, because typically some players are going for the old-fashioned Doom victory, while others are going for Citadels and the interaction becomes even more thrilling.