When bundling our application, the index.js file is used as the entry point into the React code. If we look inside this file, we can see that it imports some symbols, and then calls the render method from React DOM. This method takes two arguments. The first is the markup we want to insert into our application, and the second is the reference to an HTML element in the index.html file. 

Looking closely at this function call, you'll see that the first argument appears to be markup, and not a JavaScript expression, as you might expect it. It is something called JSX, and we'll cover that in a different video. But to use this syntax, we have to import React. What the expression actually is, is a React component called script mode wrapping our app components. 

The strict mode component was recently introduced as a tool for highlighting potential problems in our application, and it doesn't render anything. So our application will actually render the app component inside the first element in the index.html file that has the idea of route. The service worker mentioned here, is functionality baked into create React app, and it allows you to create progressive web applications using react. By default, this functionality is disabled.