Original: In Plato's account, Atlantis was a naval power lying "in front of the Pillars of Hercules" that conquered many parts of Western Europe and Africa 9,000 years before the time of Solon, or approximately 9600 BC.
Simplified: Atlantis is a name for a large island or small continent that might have been in the Atlantic Ocean a long time ago, but sank into the sea and disappeared.