The super parent for all the movieClip is _root
but using the _parent you can get particular MovieClip's parent.

For example you are having one Movie Called MovieHolder_mc
and MovieHolder2_mc on Stage, and inner1_mc inside the
MovieHolder_mc. Now you can access the MovieHolder in Two Ways


_root.MovieHolder2_mc (Direct access)

to get this from the innerMovie 

inner1_mc._parent._parent.MovieHolder2_mc

_root is used to access outer Movieclip.

_parent is used to access Moviclip inside having the 
moviclip.

_parent is used to access the parent display
objects/properties of a MovieClip/Button. its something like
the relative path to the current MovieClip/Button. _root is
like the absolute path irrespective of the location of the
current MovieClip/Button.