By combining several programming languages as mentioned above, AJAX allows for web pages to update immediately upon a user's interaction. Imagine a time you've dragged and dropped something on the Internet without the page reloading. That's AJAX. By decreasing the amount of HTTP requests needed for a user to navigate your website, your site becomes faster.
Little known tip: as your website becomes faster and is responsible for less HTTP requests, your search engine rankings will be affected positively.