In skin url means skin folder in root of the core folder of magento and if you want to explain it in the language of programming skin url means http://www.yourdomainname.com/skin
means the position of skin folder.
So, if we want find a image in a folder like //skin/default/default/images/add/XX.jpg then we just put these code to show the URL
In such way the Media URL will be
And Store URL::