Deep in the fires of Mt, Doom, Sauron took the One Ring and held it above his head and laughed with cruel glee. When he put on this Ring everyone would know him and his evil intentions. They would fight, he knew. And he was always prepared. So, just in case, he made another Ring with the same powers as the first. He would never put it on until the time was ripe, so the Elves would never know about the Second Ring.
As we all know, after a long time the One Ring was destroyed. But the Second Ring survived. At the last battle the Second Ring was flung somewhere, and Sauron did not know where it was. So Sauron's defeat was not the end of him, as many believed. But Sauron could not do anything because he did not know where the Second Ring was. So he waited, until a few years later, when the Second Ring was found.