Hernandez means "son of Hernando" (also Hernán or Fernando)—Spanish forms of a Germanic personal name that English speakers usually meet as Ferdinand. The -ez ending marks descent the same way it does in Martínez or González.
Fernández is a closely related twin spelling from Fernando. Sephardic Jewish families sometimes adopted Hernandez at conversion and kept it later; that history does not change the word's patronymic sense. For most US bearers, the short gloss is simply: son of Hernando.