Every URL you have ever seen, starts with http, but what exactly is HTTP?

MDN discusses it as such:

Hypertext Transfer Protocol (HTTP) is an application-layer protocol for transmitting hypermedia documents, such as HTML. It was designed for communication between web browsers and web servers, but it can also be used for other purposes. HTTP follows a classical client-server model, with a client opening a connection to make a request, then waiting until it receives a response. HTTP is a stateless protocol, meaning that the server does not keep any data (state) between two requests.

When you connect to this webpage to download the content, your browser creates a HTTP request to GitHub, and GitHub responds with a document your browser understands, dependent on the request that you made.