Crop/Uncrop will crop or uncrop the active movie. When you crop a movie to the crop rectangle, the portion of the movie outside of the rectangle is not deleted from the movie - it just isnt shown. Therefore, you can uncrop a movie clip to restore it to its original size and images.
The cropping rectangle and cropping status are retained with the movie when it is saved so that cropping does not have to be turned on each time a movie is retrieved a file.