Why keys only make sense in the context of the surrounding array when I extract components with keys in React?

Question

I'm a beginner in React. When I read the doc, Extracting Components with Keys, I feel confused.

Because there is a sentence, "Keys only make sense in the context of the surrounding array."

I want to know why keys only make sense in the context of the surrounding array when I extract components with keys in React?

Something I don't know or it's just a regulation?

Thanks for your answers.