Original: Urbi et Orbi ("to the City of Rome and to the World") was a standard opening of Roman proclamations. The term is now used to denote a papal address and Apostolic Blessing that is addressed to the City of Rome and to the entire world.
Simplified: Now Urbi et Orbi refers to a papal address and Apostolic Blessing that is addressed to the City of Rome and to the entire world from the Apostolic Palace in the Vatican.