Sauron is like a big, bad boss in a story called "Lord of the Rings." Imagine if you're playing a game and there's a really tough level boss that you have to beat to win the game - that's Sauron.
He's a bad dude who wants to take over the world and do bad things to everyone. He's not a nice guy - he likes to hurt people and he doesn't care about their feelings.
Sauron used to be a happy little elf many years ago, but he stopped being happy and became angry and mean. He made an object called the One Ring which makes him really powerful and it can also control people's minds.
The One Ring is his weakness, though. If someone destroys the One Ring, Sauron's power will be destroyed with it.
But it's not easy to defeat Sauron - he's got a big army of bad guys working for him, and many people have tried to beat him but failed.
So, in the end, the little hobbit Frodo Baggins with his friends Sam, Merry and Pippin, was the only one brave enough to try and destroy the One Ring and defeat Sauron. And they succeeded, thanks to their courage and friendship.