This line means something like, “Make ready the steel and the bridle.” It continues the battle imagery from the first line. The anthem is telling Mexicans to prepare weapons and horses, the way soldiers would prepare for combat in the era when the anthem was written.

Acero literally means “steel,” but here it stands for a sword or weapon made of steel. Aprestad is an old-fashioned command form from aprestar, meaning “prepare” or “make ready.” Bridón means a bridle, the headgear used to control a horse, and by extension evokes cavalry. The line sounds strange in modern Spanish because it uses formal, poetic vocabulary rather than everyday speech.