AJAX, or Ajax, is a web development technique used for creating interactive web applications. The intent is to make web pages feel more responsive by exchanging small amounts of data with the server behind the scenes, so that the entire web page does not have to be reloaded each time the user requests a change. This is intended to increase the web page's interactivity, speed, functionality, and usability.
POSTing large quantities of data (e.g., 2,200 characters in length) to a script running on an HTTPS server, with Extra Security enabled, may result in an HTTP 503 error ("Service Temporarily Unavailable"). This error is generated by mod_security - see that article for instructions on disabling it for specific scripts.