Lennie is marginalized in society on account of his learning disabilities. A grown man with the mental age of a child, Lennie is one of society's outcasts. Society in 1930s America didn't really understand disabilities all that well, and Lennie suffers from a general lack of comprehension among the people he encounters.
He and George are forced to leave the town of Weed after Lennie starts stroking a girl's dress. Lennie doesn't mean anything by it; he just likes stroking soft objects. But the girl is understandably shaken by the incident, and so it's time for Lennie and George to leave town before they get into any more trouble.
But the marginalization of Lennie continues after he and George wind up at the ranch. One day, Curley takes his frustration out on Lennie, picking on him because of his learning disabilities. Unfortunately for him, Lennie proceeds to crush his hand, leaving him in considerable pain. That Curley instinctively felt no compunction in picking on Lennie in the first place, however, indicates the lowly status that the disabled have in this society.
Tragically, Lennie's marginalization ultimately leads to his death. Once again, he doesn't know his strength, and after Curley's wife pulls away from Lennie after he starts stroking her hair, he inadvertently kills her. Lennie takes off and is hunted down by the local posse like he's some kind of wild animal. This is a further sign of his outsider status in this harsh, unforgiving society.