Whenever your browser goes to a web site, the very first information that is returned for every page you visit is the HTTP header. The data in the header tells your browser a little bit about the page itself, like what kind of content is contained in the page, how big the page is, the kind of server being used, the protocol used (http or https for instance), cookie data, any form post data, etc. This information is processed by your browser to help it properly display the page.