Skins are basically CSS styling on the DOM structure that represents the editor. Fortunately the world is going into a direction where browsers are aligning their CSS features to standards, which makes it easier to design CSS that works everywhere.
But still the world is not perfect and we have small differences on CSS among browsers. Additionally, CKEditor must support ancient browsers, which are more limited and buggy.
To make it easier to maintain the skin CSS, CKEditor makes it possible to define browser specific files, which hold all "hacks" necessary for them. For example, a skin can contain the editor_ie.css file with all IE hacks or dialog_opera.css for Opera specific stuff.
A skin must instruct CKEditor to load, for example, editor_ie.css instead of editor.css on IE browsers. This must be done by setting the CKEDITOR.skin.ua_editor value to the list of "browser files" available. The same can be done for dialog.css. Check out the skin.js file of the Kama skin for a real example.