The common scenario to reach the desired page on the internet is to write the URL to a browser and press Enter. Basically say the browser asks a server for the requested page. The server builds the page (HTML) and sends the result back to the browser. The browser displays the result to the user and makes HTTP requests to other content like images, etc. Let’s jump back to the stage when the server is building the content of the page. Some parts of the page are usually static data immediately available to use. Another data of the page could be dynamic, either from huge database heavy for calculation or from another server like RSS channel. The point is that some of the data is quickly available and some of the data need time to be loaded. The HTML build by server isn’t sending to the browser until it is completely created. That is obvious.
Hope this will help