Well firstly if you want to know you should read the books!
But briefly, "the rings" stands for the rings of power made a long long time ago by the elves. 3 for the elven kings, 7 for the dwarf lords, 9 for kings of men. "the one of ring" was made by sauron to rule them all. That's why he's the lord of it. And if he obtained it he will become the lord of all rings.
The 9 kings have become the ringwraiths so sauron has them already and the 7 from the dwarves. Only the 3 elven rings eludes him. I hope that explains "the rings"