Wikipedia
Hochgeboren is a form of address for the titled members of the German and Austrian nobility, ranking just below the sovereign and mediatised dynasties. It can be translated into High-born or of High Birth.
The actual address is "Euer" Hochgeboren. It is the proper form of address for counts (Grafen) that are neither heirs to mediatised families of the Holy Roman Empire ( counts of the Holy Roman Empire or Reichsgrafen) nor families who have been bequeathed higher predicate by the Emperor. By courtesy, barons (Freiherr) belonging to old houses of the Uradel are also addressed in the same way.
The correct term for immediate counts (Reichsgrafen) is Erlaucht ("Illustrious Highness"), while the proper form of address for princes (Fürsten) and dukes (Herzöge) is Durchlaucht ("Serene Highness").