The best excerpt that summarizes what Steinbeck's Of Mice and Men is about is to be found in the first chapter, where George is telling Lennie about the dream they both share.
"Guys like us, that work on ranches, are the loneliest guys in the world. They got no family. They don't belong no place. They come to a ranch an' work up a stake and then they go inta town and blow their stake, and the first thing you know they're poundin' their tail on some other ranch. They ain't got nothing to look ahead to."
This encapsulates what the story is about. Steinbeck's novella features George and Lennie, but they are representatives of all the itinerant farm workers that George refers to as "guys like us." The fact that George and Lennie share a dream of having their own little farm provides motivation and plot, but the author's purpose is to present a compassionate portrayal of a whole class of working men who have to toil for a bare existence and have no security in the present and no hope for the future.
In the last chapter, when George is preparing to kill Lennie, he repeats some of the same story he has been telling his friend for a long while:
"Guys like us got no family. They make a little stake an' then they blow it in. They ain't got nobody in the worl' that gives a hoot in hell about 'em--"