George and Lennie are both migrant workers eeking out a living as farmhands in California during the 1930's. They live in constant precarity and as drifters they are unable to settle down and "belong" anywhere.
George has known Lennie since childhood and has taken on the responsibility of looking after Lennie ever since his aunt died. They are friends by circumstance, but their friendship goes deeper than just that. Unlike other migrant workers, George and Lennie have each other to count on; they are "family."
This is why their dream of owning and running a small farm is so important. As much as financial security, George and Lennie need each other's companionship and roots.