The Tortilla Curtain is a novel dealing with the disconnect between the Mexican and U.S. cultures, and the title needs to be understood in its historical context.
During the Cold War the term "Iron Curtain," coined by Winston Churchill, referred to the boundary between Eastern Europe—at that time the Soviet Union and its "satellite" countries such as Poland, Hungary, and East Germany, upon which communism had been imposed by the Soviets—and the democratic Western European states (West Germany, France, Britain, and so on). The boundary was both a physical and an ideological one, and was a barrier across which many Eastern Europeans attempted to flee in order to escape communism and reach the democratic West. The policy of the Soviet bloc countries was in effect to keep their citizens imprisoned and to forbid immigration to the West, and hundreds of kilometers of barbed wire and other physical barriers such as the Berlin Wall were set up to prevent people from escaping the communist states.
T. C. Boyle, in transmuting "iron curtain" into "tortilla curtain," is analogizing this communism-democracy dichotomy with the divide between Mexico and the United States. Just as people fled the communist countries during the Cold War, people leave Mexico for the United States in search of jobs and a better mode of living. But the larger point is that the split between the latino and U.S. cultures exists within the States, since in the novel Candido and America live practically adjacent to the Mossbachers in the Los Angeles area, but under completely different economic conditions. So a curtain exists as both a physical border between Mexico and the U.S., and as a cultural divide among people of different ethnicity—just as, during the Cold War, there was both a physical and ideological divide between Communism and the "Free World."