Lennie has what we would now call a learning disability. He has the mental age of a child, and even with George there to look after him, this still frequently gets him into serious trouble. But despite his disability, Lennie has proved time and time again that he is a hardworking man more than capable of making a contribution to the operation of the ranch. What he lacks in intellectual capacity, he more than makes up for in physical strength.
Candy is physically disabled. He lost a hand in an accident years before and has been working as a swamper ever since. This is a menial job, to put it mildly, and it involves cleaning out the bunkhouse while the men are out working on the ranch. Steinbeck shows Candy as being hardworking and conscientious despite his serious disability. He has managed to cobble together some savings over the years, which allows him to dream of joining George and Lennie on their ranch.
In the characters of Lennie and Candy, Steinbeck humanizes disabled people and shows his contemporaries that they have abilities that society often overlooks. He also shows that they have dreams and ambitions just like everyone else.