Every thing in a web is based on request and response. If you want something you are requesting it, where do you do it, it is nothing but your browser.
The request is sent to the server. The server fetch the data and sends it back to the browser. The response can be categorized in to many status code.
Here is the categorization
100 series - Informational
200 series - Successful
300 series - Redirection
400 series - Client Error
500 series - Server Error
This will explain what is the response from server.