HTTP stands for "Hypertext Transfer Protocol". The entire World Wide Web uses this protocol. It was established in the early 1990′s. Almost everything you see in your browser is transmitted to your computer over HTTP. For example, when you opened this article page, your browser probably have sent over 40 HTTP requests and received HTTP responses for each.
HTTP headers are the core part of these HTTP requests and responses, and they carry information about the client browser, the requested page, the server and more.