The Holy See (Latin: Sancta Sedes, Italian: Santa Sede) is the episcopal jurisdiction of the Catholic Church in Rome. The primacy of Rome makes its bishop the worldwide leader of the church, commonly known as the Pope. Since Rome is the preeminent episcopal see of the Church, it contains the central government of the church, including various agencies essential to administration. Diplomatically, the Holy See acts and speaks for the whole Roman Catholic Church. It is also recognized by other subjects of international law as a sovereign entity, headed by the Pope, with which diplomatic relations can be maintained."
It is a separate entity from the Vatican City.
More information on the Wikipedia page [1].
The website, in Italian, is [2]