HTTP stands for Hypertext Transfer Protocol. If you think of the internet as a map, then HTTP is like putting an address into the GPS. Your web browser (for example, Chrome) uses those directions to get the site from the web server and show it on your screen.
Between 1999 and 2015, HTTP/1.1 was the main HTTP protocol used to navigate the World Wide Web. In 2015, HTTP/2 was introduced and significantly sped up authentication, DNS queries, and the overall routing and caching of HTML files.
You can read more in our guide to HTTP/2 and how to enable it on your WordPress site.