I'm trying to get an actionscript code used in a Flash CS5 example and make a ActionScript class using it in Flash Builder. And I'm trying to understand the `_root` property.

[This page](http://www.adobe.com/support/flash/action_scripts/actionscript_dictionary/actionscript_dictionary633.html) have a brief explanation about the `_root` property, but it is too vague.

My question is: What is the "\_root" that exists only when the ActionScript is placed in the Flash CS5 project? How do I reference it in my class in Flash Builder? (it is an instance of what?)