FlexSprite

FlexSprite is defined like this "public class FlexSprite extends Sprite". As far as Sprite goes, the docs quote that

" The Sprite class is a basic display list building block: a display list node that can display graphics and can also contain children.

A Sprite object is similar to a movie clip, but does not have a timeline. Sprite is an appropriate base class for objects that do not require timelines. For example, Sprite would be a logical base class for user interface (UI) components that typically do not use the timeline.

The Sprite class is new in ActionScript 3.0. It provides an alternative to the functionality of the MovieClip class, which retains all the functionality of previous ActionScript releases to provide backward compatibility."

It would be handy to know the difference between movieclip, timelines and Sprite, but thats for an another post.

And FlexSprite adds extends Sprite to override the toString() method. It overrides the toString() method to return a string indicating the location of the object within the hierarchy of DisplayObjects in the application.

It iterates through the parent heirarchy untill it reaches the stage object and returns the object location heirarchy.