So imagine you're watching a movie about a group of scientists who find a weird alien thing in the snow while doing research in Antarctica.
The alien thing is like nothing they've ever seen before. It can change its shape to look like any organism it has come in contact with. It's a bit like a chameleon that can change into anything.
When the scientists find this thing, they initially think it's all good and they even call it "the little guy" at first. They think they can study it and learn from it.
But then they realize that the thing is not so friendly after all. Whenever it is threatened, it goes into survival mode and defends itself by attacking and assimilating other organisms. It basically takes over their bodies and becomes them.
The scientists start getting paranoid and don't know who to trust, because the thing could be anyone or anything. They all have to work together to figure out how to stop the thing from spreading and taking over the world.
So in short, The Thing is a movie about a really creepy alien that can take on any form and the humans who try to stop it from destroying them.