George and Lennie's plan symbolizes the American Dream. It represents the idea of being able to own a piece of property and live comfortably on it. George and Lennie are both outsiders and drifters. They do not have family and friends, aside from each other. They want to build their own place where they will feel safe and free to keep animals and grow their own food, for the sole purpose of sustaining a simple lifestyle. The specific words "fatta the land" show that their dream is so innocent, that they really want nothing more than a small space of land, and they can live happily just with that.
There are various conflicts that arise throughout the story (the killing of Candy's dog, Lennie's accidental killing of Curley's wife) that foreshadow the fact that this dream is not, in fact, possible for them. Steinbeck is trying to communicate the fact that the American Dream is not achievable for many people, as it claims to be.
Keep in mind that this book takes place during the Great Depression (1930s), a time when many Americans, some who had recently immigrated to this new land of opportunities, were having to face the shocking realization that it doesn't necessarily matter how hard you work, you may never be able to achieve the American Dream. And even if you are able to achieve it, you may be forced to give it up at a moment's notice.