What AJAX allows you to do is just update parts of the DOM of a HTML webpage instead of having to reload the entire page. AJAX also lets you work asynchronously, meaning your code continues to run while that part of your webpage is trying to reload (compared to synchronously which will block your code from running until that part of your webpage is done reloading).

With interactive websites and modern web standards, AJAX is gradually being replaced by functions within JavaScript frameworks and the official Fetch API Standard.

What is Ajax? - Video
What is Ajax? – Video