Royal Highness (abbreviation HRH) is a style (His Royal Highness or Her Royal Highness); plural Royal Highnesses (abbreviation TRH, Their Royal Highnesses). It appears in front of the names of some members of some royal families, but is not used for Kings or Queens, who are styled Majesty.
The style Royal Highness ranks below Imperial Highness, but above [Highness]], Serene Highness and some other styles.