A headless browser is one that has no address bar, buttons, toolbars, etc. and it’s used to test and automate apps. In this case, Phantom JS, a test tool that uses such a browser, was used by attackers to generate 700 million hits per day on the targeted site. Read more here:
http://m.darkreading.com/133696/show/dcc2e78b8f13399672bc73e5b26fe4aa/