Asynchronous JavaScript and XML (AJAX) was the base building block of Single Page Applications (SPAs). This method is used to update the content of a web page without reloading the whole page. This method helps save precious resources and decreases the time taken for loading a page considerably, since only parts of the page are reloaded and not the entire page.
More than often, you have visited the Google search page for searching answers to your questions. Have you noticed how the page displays results as you type into the search box and provides useful suggestions of related searches? Or the product filters on Amazon and Ebay websites. These effects are achieved with AJAX.