A part/section of information sent by web servers (HTTP headers) along with your files.
A Web server's job is basically to listen on a given port for incoming HTTP requests and to generate appropriate HTTP responses. When you type your server's URL into a browser's location box, the browser sends an HTTP request to the server and waits for the response. Typically, if the request is for a simple HTML file, the browser will get back a bunch of HTML and render it.