The image of the rabbit in Of Mice and Men is a symbol for George and Lennie's dream to own a ranch and "live off the fatta' the land." Throughout the novel, Lennie asks George to recount details of their dreams, and each time, Lennie focuses on his own role to tend the rabbits. Because Lennie focuses so much on this aspect, the image of the rabbits conjures images of the ranch and their dream.
Rabbits could also represent, on a bigger scale, failed dreams. George and Lennie have been working toward their dream to own their own ranch and look after themselves. They have to revise and adapt this plan as they go because Lennie continues to cause problems for them. In the first chapter, we learn that they had to leave their previous town because Lennie wanted to touch the soft fabric of a girl's dress, and the people misjudge his intentions and they are run out of town. This theme of failure is repeated throughout the novel, and in the end George and Lennie fail to reach their dreams. Because the rabbits represent their dreams, especially for Lennie, they also represent the failure of those dreams.