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.