ARH, or the Ability Requirement Hack, is a hack originally created by Xifanie and then re-posted by Pokeytax. It allows the user to define conditions that are required for an ability to be used during battle, much like the old "requires sword" and "requires materia blade" functions. However, the required conditions can include status effects, equipment, gender, job, and more. If the requirement is not met, the ability will not appear in the list of usable abilities during battle.
For more information, read this thread:
Note: does anybody know if there was an original thread for this hack before Pokeytax re-posted it? If so, please post a link to it here if it is helpful.