From Nookipedia, the Animal Crossing wiki
The Dream Suite in a building introduced in Animal Crossing: New Leaf, located on Main Street. The Suite is run by Luna, who will allow the player to visit friends' and random players' towns by dreaming.
The Dream Suite is not initially available and can be constructed as a public works project. Seven days after becoming mayor, the player may walk into Town Hall and find Isabelle sleeping. Upon waking her up and talking to her, she will say she had a dream and suggests that the Dream Suite be opened. After paying Lloid 234,000 Bells at the Train Station, the Dream Suite will open the next day.
 Sharing a Dream
Once a day, the player can share a dream of their own town with other New Leaf players by lying down on the couch. Luna will ask the player whether or not custom designs from the town can be taken by visitors; if the player allows this, Wendell will appear in dreams of the town to provide the designs to dreamers. Luna will then tell the player with their town's Dream Address and pay the player 5000 Bells.
The first time the player uploads the dream of their town, Luna will add the town's Dream Address to the player's Town Pass Card. After later updates, Luna will tell the player how many other players have dreamed of their towns. The player may also talk directly to Luna to have the dream of their town deleted.
To begin a dream, the player must lie down on the bed and pay Luna 500 Bells. They may then choose to visit a random town, input the Dream Address of a specific town to visit, or search for a town. They may then choose to visit a previously visited town or a random town, or to input the dream address of a new town to visit. While dreaming, the player may walk around the town and perform actions just as they would in the real world, but their actions will have no effect on the town.
The player cannot go to Main Street or enter any buildings with doors besides homes. Additionally, messages left on the bulletin board cannot be read; instead, the board displays the town's name and Dream Address. If the player who uploaded the town chose to allow their custom designs to be taken, the dreaming player will be able to talk to Wendell to access the designs. Regardless, custom designs on display in the town, such as on the ground and in houses, will be visible. The player who uploaded the town can also be found walking about. When spoken to, they will say their recorded greeting.
Upon returning from a dream selected by Luna, if the player tells Luna that they had a nightmare, Luna will not send the player to the town again.