The United Nations is the common name for an otherwise unnamed goverment organization from the Marvel Cinematic Universe. "The United Nations" was the prominent goverment organization after S.H.I.E.L.D.'s fall during the events of Captain America: The Winter Soldier. "The United Nations" was the second major instigator during the Civil War (the first being Helmut Zemo), as it forced superheroes to submit to its authority. After the airport fight, those who fought to capture Bucky Barnes were immediately signed into the Accords and allowed to act freely, while those who fought to protect Bucky were considered enemies of "the United Nations" and ruthlessly hunted down by them (unless the Pro-Bucky superheroes managed to get a deal for house arrest).
The organization still tried to hunt down the Pro-Bucky superheroes during Thanos' invasion of Earth.
The organization is still active after Thanos' death and the resurrection of half of the universe's population.
The organization is led by Thaddeus Ross, presumably as a way for him to atone for his actions during the events of The Incredible Hulk.
Despite what the organization is commonly known as, it has no relationship to the real-life United Nations (which exists as a separate entity in the MCU).