HTTP is short for Hyper Text Transfer Protocol. It defines how data is sent back and forth across the Web. There is a "client" computer that requests, for example, a file from the "server" computer which then puts in motion an exchange in communication.
This exchange may take place with you entering a website address (or URL) into your browser which informs the browser to send a request across the web to a web server. The web server receives and processes the request using a web server application then returns a response back to your browser. If the response is good the browser will start downloading the web page, if not, the browsers will return an error page.
Here's an example of a page that cannot be found on the Google website. The browser has requested the locations of a page https://google.com/cant-find-this that does not exist on the Google server. So a friendly error response is displayed to the user and in the background also the browser. In this case, the error response code was 404. In the case of a successful response, a 200 code will be returned to the browser.
To get a good overall rundown on HTTP to see this page on the Mozilla website. It steps things out quite well.
Here's a YouTube video resource from Dave Xiang that helps to understand HTTP.
What is HTTP?
Author: Dave Xiang
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