It is the protocol used to transfer data over the World Wide Web. It defines how messages are formatted and transmitted and what actions should be taken by the web servers and browsers in response to various commands.
e.g. when you enter a URL in your browser, this actually sends a HTTP command to the web server directing it to fetch and transmit the requested Webpage. The Other main standard that controls how the World Wide Web work is HTML, which Covers how web pages are formatted and displaued. HTTP is stateless protocol because each command is executed independently without any knowledge of the commands that came before it.