If you’ve ever used a server-side language like PHP, ASP.NET, etc, you have heard about the use of “dynamic languages.” The idea is that you tell the server what you want and, instead of doing all processing in memory, the server only creates an output buffer and sends the response to the browser as text. This way, the server processor only has to process the response once.
This is where AJAX comes in handy. AJAX is one of the most useful things that exists in the browser, it lets you send multiple requests to the same server at once, and then all of them can be sent back and processed by the browser. Basically, it lets you do a bunch of requests to one server and then send them all back to the same client.
One of the reasons AJAX is so useful is that it allows you to make multiple requests to the same server at the same time. When you make multiple requests and the server responds with the same information (say, a bunch of images for your site) then you know that your site is working correctly. AJAX allows you to send multiple requests to the same server at the same time, and then the browser can process the requests back to you.