Component : a piece of reusable code that represents a part of a user interface.
JSX element : a combination of JavaScript code and HTML tags that describes what you’d like to display.
Index.js: the bridge between the component you created in the App.js file and the web browser.
To “remember” things, components use state.
useState : a special function that you can call from your component to let it “remember” things.
To collect data from multiple children, or to have two child components communicate with each other, declare the shared state in their parent component instead.