Read more about Javascript atw3schools.comandWikipedia. You essentially have total access to the Javascript used to manipulate your browser because your browser has to download it in the first place before the
Here we are taking the document object (which is given to us for free by javascripthttp://www.w3schools.com/jsref/dom_obj_document.asp). Thedocumentobject is a reference to our whole page. In the console we can run > document.write('foo'); ...