https://fpt-software.udemy.com/course/build-a-serverless-app-with-aws-lambda-hands-on 

CORS - Cross origin resource sharing

AJAX asynchronous JavaScript. Only fetch data from the same page it call to. Why?

Because you don’t want the website send your personal data to other website?? Not clear yet??? Prevent the website to make unauthorized request to other website on behalf of the user.