criollo is a Spanish word that means "person born of entirely Spanish stock, but outside Spain". The criollos in Mexico became the ruling elite with the Independence War. Contrast with mestizo and indio.
Currently the word criollo is not in general use in Mexico.
adamk (thank you!) observes also that in South America criollo primarily means "del pueblo" and provides a Peruvian example: "este gringo es más criollo que la chicha morada", meaning "this gringo speaks Spanish and acts like someone from here".