George and Lennie play on saving their money ("jack") until they have enough to buy a piece of land. They intend to farm the land and raise animals such as chickens and rabbits. This dream is what keeps them going from job to job and it is what keeps their hopes up. George remarks that most itinerant ranchers spend their money frivolously, whereas he (George) and Lennie have a goal and that goal prevents them from wasting their money.
With us it ain't like that. We got a future. We got somebody to talk to that gives a damn about us. We don't have to sit in no bar room blowin' in our jack jus' because we got no place else to go. If them other guys gets in jail they can rot for all anybody gives a damn. But not us.
Not only do George and Lennie have a goal/dream that motivates them to save their money; they also have each other. Many itinerant ranchers are loners and are responsible to no one but themselves. But George and Lennie depend upon each other and because of that interdependence, their friendship is like a familial bond.
The problem is that Lennie's poor social skills have made them flee their previous job and when he makes a much larger mistake at their current job, their dream of owning a farm together comes to a tragic end. So, they never raise enough "jack" to buy the land.