On a personal level, in Of Mice and Men, Lennie and George are misfits: Lennie because of his mental state, George because he takes care of Lennie.
Ironically, Lennie is aware he causes problems in a vague way, but really doesn't understand his behavior and is powerless to stop it. George understands the behavior, and spends much of his thought and actions trying to prevent it, but he is also powerless to stop it. As a result, they are not welcome anywhere for very long, and saying "not welcome anywhere" is an understatement, of course.
The dream of the ranch and the rabbits is a dream of a place where they would be always welcome. Lennie could pet rabbits all he wants, and George would not have to worry about Lennie.
Of course, in keeping with the economic situation in the novel, and by extension in the U.S. and other places, of course, this dream is an illusion. It's illusory not only because of economics, however, but also because society has no place for someone like Lennie: society has no means to treat or accept or maintain someone in Lennie's mental condition.
In this story, Steinbeck is trying to talk about the American Dream. The ranch is the symbol of this dream.
The major thing that Lennie and George would have if they got to the ranch would be independence. So Steinbeck was emphasizing that part of the American Dream was the desire to control one's own destiny. On the ranch, George said, the two of them would be able to do whatever they wanted. They would not have to answer to any bosses or ask permission for anything.
Of course, this dream is not realized because Steinbeck is saying that America's society at the time makes it impossible for people like George and Lennie to live the American Dream.
The ranch in the story Of Mice and Men is the embodiment of the American Dream for the two men. They have been drifters like so many men like them during the depression era. Many men wandered in and out of ranches and farms looking for work. They would spend their days working hard and on weekends they would blow their earnings drinking it away.
George and Lenny have a dream about having a ranch of their own. For the two of them it means stability and safety for Lenny who often does things without knowing he has done something wrong. George often tells Lenny that it is what makes them different, their seeking ownership of their own place.