HTTP means Hypertext Transfer Protocol. It is an Application Layer Protocol over which information is sent to your web browser to the website, that you browse. The browser and the website communicate with each other through data, which is sent in plain text. The problem with plain text is, that it can be intersected and read by others.
Therefore the HTTP protocol has been updated to a more secure version, called HTTPS; Hypertext Transfer Protocol Secure. When the browser requests a webpage it will get an HTTP status code in return. If you request a specific page of a website that does not exist anymore you might get a 404 status code in return - but in fact, there are 65 different HTTP status codes.