Re-skinning an app means that you replace all the image, sound, and creative assets with your own in order to make a "new" app. You don't have to change any of the code if you don't want, but usually you'll want to update IDs for ad networks and in-app purchases.
Think of it like you are buying a template that you need to add art to. This model is in many industries. In fact this website is a template of the Thesis theme on Wordpress that has been re-designed.