“Hispanic” means relating to Spain. So, “pre-hispanic” means “before Spain showed up.”
Another word for this is “pre-Columbian,” which means “before Columbus came over to the New World.”
(Those aren’t technically the same thing, but they roughly coincide.)
Spain conquered the Aztec empire in the 1500. So “pre-Hispanic Mexico” refers to the ancient kingdoms of Mexico: the Aztecs, the Mayans, the Incas, etc.