This is one of the greatest features with visual studio when working with Silverlight. It allows you to jump or navigate on the XAML easily. If your Silverlight application has thousands of tags on the page, then you could easily get lost in the XAML code. This feature is very handy in these scenarios.
This window can be opened by clicking on the icon located in bottom left corner of Visual Studio:
Or else you can go to View Menu -> Other Windows -> Select Document Outline menu item. You also can use the short cut key combination Ctrl+W,U.