The skin refers to the 'look and feel' of your website. It consists of a set of .css files created in a specific folder. Each layout uses a skin. Normally all the layouts in a website use the same skin for a standard design. But you could create more than one skin.
For Zenario, a 'skin' is not other thing that a folder containing a set of files (fonts, images, styles, settings) that create the full look and feel of your website.
Here are some examples on how having this file improves the interaction of the skin with Zenario.
Zenario comes with a default skin as standard which has a simple, clean look. Should you want to change this look it is possible to extend the skin using this method.
The layout is a key part of the HTML structure of a website. View this guide on how to set the skin of a layout.