Sheol is a place that people used to believe existed a long, long time ago. It was kind of like a big, dark hole in the ground where people would go when they died. In this place, they thought people were just sort of sleeping or waiting until something else happens.
Imagine a big, spooky cave where everyone who died went to. But don't worry, because people don't really believe in Sheol anymore. It was just an idea that people had a very long time ago.