With this feature you can add the current url of the iframe to the parent. This enables bookmarkable urls where you go directly to the last page in the iframe. So if someone clicks inside the iframe the new url is added as parameter to the parent url and if you use this url the page in the iframe is not the default one but the one from the parameter. The history api which enables the change of the url is only supported by modern browsers. For older browsers the url is simply not changed. See http://caniuse.com/#search=pushstate.
There are 2 examples for this feature: