The answer to this question is dependent on your place in the novella.
In chatper 1, the dream remains a dream. George uses it to keep Lennie focused for their future.
By chapter 3-4. Lennie and George have shared their idea with Candy. At this point Candy has some money saved up and the dream looks like a possible reality for the first time. They further shape ideas about who will have what chores.
By chapter 6, it is obvious that the dream will never be realized. As George works to take care of the damage that Lennie has caused, he realizes there is only one answer to never having to deal with Lennie's mistakes again. George again uses the dream to focus Lennie just before he takes Lennie's life. In this moment, it is as if Lennie is actually going to that place because George is putting the vision of it in Lennie's head.