Royal Highness is a style used to address or refer to some members of royal families, usually princes or princesses. Kings and their female consorts, as well as queens regnant, are usually styled Majesty.
When used as a direct form of address, spoken or written, it takes the form Your Royal Highness. When used as a third-person reference, it is gender-specific (His Royal Highness or Her Royal Highness, both abbreviated HRH) and in plural, Their Royal Highnesses (TRH).