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.

 Ash nazg durbatulûk, ash nazg gimbatul, Ash nazg thrakatulûk agh burzum-ishi krimpatul" 
 These lines refer to the One Ring of Power on which the  Lord of the Rings  trilogy centers, and are inscribed on the One Ring itself in the Black Speech of Mordor. The inscription glows when the ring, which otherwise appears featureless, is heated. Tolkien was an Oxford philologist and linguist with a passion for inventing languages; the Black Speech of Mordor, like the other languages of Middle-earth, is an artifact of his own creation. 
 The One Ring is the most powerful of all the Rings, able to control those who wear the others. It boasts a number of magical properties: those who wear it become largely invisible to ordinary beings, like Men, but  more  visible to the Nazgûl (or Ringwraiths, the nine Men who succumbed to the Ring’s corruption and became bound to its power). It can conform to any finger and is impervious to destruction except in the fires of Mount Doom, where it was forged. It dims the wearer’s sight (perhaps symbolic of its moral effect), while at the same time sharpening the other senses. Inevitably it corrupts the wearer, gaining more and more power over his psyche in a process reminiscent of the phrase “absolute power corrupts absolutely.” 
 The trailer for Part I of Peter Jackson’s  Lord of the Rings  film adaptation provides a handy pocket dramatization of the story behind the Ring: