What is your opinion on what the story is trying to tell the readers?

"The Tortilla Curtain" is based on the consequences of immigrants jumping the "wall" of the Mexico-U.S. border. This includes both physical walls and literal walls; walls that are meant to keep things out, as well as walls built to keep things in. 

Gradesaver's page "About The Tortilla Curtain" gives a detailed look at Boyle's intentions. I have provided the direct link below.