HTTP is the communication protocol that happens between your web server and the user's browser. Without too much detail, this is broken into two pieces of data: headers and the body. The body is the HTML you send. But, before the body is sent, the HTTP headers are sent.

He includes a sample raw HTTP response for a page and how the ob_start function works to buffer the output of the resulting page to save the header information until the buffer is echoed or cleaned out. There is a down side he mentions, though - there's no partial buffering built in so it's an all or nothing short.