This property yields an object that represents the frame that contains this window object. This is the same value that the window property yields. It is another gratuitous extension that MSIE provides so that scripts to run inside frames can look more consistent with what they are doing and yet break on any non-MSIE browsers due to the frame property not being present.
Frame objects support all the methods and properties of window objects although they may not always be meaningful in the context of a frame living in a frame-set.
Of course, you can emulate this in other browsers by assigning the value of the window.window property to the window.frame property anyway.
This appears to be undocumented and does not appear in the Microsoft reference information. Nevertheless, the property is visible by enumeration of the window object and appears to work.
Using this property renders your scripts non-portable unless you code around the missing property on non-MSIE browsers.