To undertand the  HttpKernel first we need to understand how the web page works over the server.

Every HTTP web interaction begins with a request and ends with a response. 

for this we require  server and browser.

Generally developer make a request (create PHP code)(e.g. the URL)  in a browser;

The browser sends a request to the server;

The server sends back the 

to the browser;

The browser displays the resource to the user.

Generally developer make a request (create PHP code)(e.g. the URL)  and returns a response (e.g. an HTML page or JSON string).