What is a thesis in simple language?

Imagine you're writing a paper about why cats are awesome. Your thesis is like the main argument you're making:

"Cats are the best pets because they are independent, affectionate, and low-maintenance."

That's your main idea, the central point you're trying to prove throughout your paper. Everything you write will be supporting that idea.

So, in simple terms:

Thesis = Your main argument in your paper.