What can we say? It seems pretty straightforward: Tortilla Flat is about, well, Tortilla Flat. Right?
Well, yes, but it's not quite that cut and dried. By focusing on the name of a community, Steinbeck is trying to show the way an entire community works, how the relationships in it are built, and how this community and its relationships are tied to a particular place—in this, Tortilla Flat.
The community of Tortilla Flat is defined by not being considered a part of Monterey proper; it's on the outskirts of town, and the people who live in it are outsiders. Steinbeck tells us that Tortilla Flat "isn't a flat at all," (Preface.5), which gives us an indication that it's a misunderstood place (and the people in it are probably misunderstood as well). The novel's job is to do Tortilla Flat and its inhabitants justice, and maybe to help us to understand them a little better.