HRH is a written short form of His Royal Highness or Her Royal Highness. It is a title you put before the name of a prince or a princess, in very formal English: HRH The Princess. You see it in newspapers, palace notices, and photo captions. You would not say “HRH” to a friend about your teacher. Which word you expand — His or Her — depends on the person. Though actually, this is not the same as Majesty, which is for a king or queen; mixing the two sounds wrong in careful writing. People write the letters; they rarely say “H-R-H” in ordinary talk.