According to a Texas A&M University website, the Mexican flag stands for Mexican values and historical tradition. The three colors of the flag were the colors of the Army of the Three Guarantees, which fought for independence from Spain.
Originally, the green represented independence, the red represented union between Europeans and Americans and the white represented the Catholic faith. The symbolism of those colors has changed, though, according to About.com. Today, the white stands for unity, green stands for hope and red stands for the blood of the national heroes. The seal in the center of the flag depicts Mexico's founding story. In this tale, the Aztecs, who were also called the Mexica, were searching for a home. The god of war, Huitzilopochtli, told the Aztec leader to build their city where they saw an eagle eating a snake on a prickly pear cactus. They did just that, and their city, Tenochtitlan, is now known as Mexico City.