In the beginning of the book, George and Lennie are traveling to their next job. They stop to drink on a riverbank. George complains about the burden Lennie is, but also says that they need to stick together.
Lennie has a mouse that he has been "petting" that died. This foreshadows later incidents in the story involving Lennie's innocent petting , like the death of the puppy and Curly's wife. George tells Lennie he can't have the mouse, but Lennie keeps it secretly. George entertains Lennie with stories about the rabbit farm that they will have some day, where Lennie can pet the rabbits.
The rabbit farm is symbolic of the unreachable dream. George and Lennie will never be able to live successfully and peacefully, because they can never get ahead as migrant labor. Lennie will continue to be a burden on George, and George cannot keep Lennie safe or protect others from Lennie. This is exemplified by Lennie drinking unsafe standing water, and the mouse's accidental death.
This opening also demonstrates that while George berates Lennie and says he is a burden, he also is careful to look out for Lennie. His spinning the story shows both that he cares for Lennie and that he realizes that they will never achieve their dream.