In the stories, there are magical rings with various powers that were created by an evil lord/sorcerer to corrupt the leaders of Middle Earth so that he could take control of the entire land.
"Three Rings for the Elven-kings under the sky, Seven for the Dwarf-lords in their halls of stone, Nine for Mortal Men doomed to die, One for the Dark Lord on his dark throne In the Land of Mordor where the Shadows lie. One Ring to rule them all, One Ring to find them, One Ring to bring them all and in the darkness bind them In the Land of Mordor where the Shadows lie."