This story is about how we are all prisoners of circumstance, and how what seems like the chance to escape the misery of our lives can be but an illusion. It shows how something which in an ideal world should be a great blessing can in fact become a curse. Such is the discovery of the `Pearl of the World' for Kino and his family.
The darker side of human nature is very much paramount in this story, revealing to what depths people will go for the sake of their own financial gain. Steinbeck uses the story of the pearl to illustrate how difficult it can be to change the course of our lives, and how if we try to break out of the unwritten consensus which governs our daily lives, things can not only become lonely but also dangerous, as Kino discovers to his great cost.
I'm not generally a fan of short stories, but this one says more about human nature than some authors can fit into 400 pages. As usual with Steinbeck, it is a very good piece of writing.