This is a tool used to make skinning a SharePoint site via css or a custom theme much easier.
If you have attempted to override the styles on an out of the box SharePoint site, you know that it isn't a very easy thing to do.
The core.css file that contains the basic rules has 979 different style rules.
The core.css file uses a palette of 132 colors and 143 images.
The default page of a newly provisioned team site uses only 61 of those rules.
So, unless you have branded a lot of SharePoint sites and are intimately familiar with core.css and the default master pages and page layouts, just figuring out where to start modifying can be a daunting task. With SharePoint Skinner you can create new styles by altering existing styles in a WYSISWYG fashion. If you are not expert in writing CSS try complimenting Skinner with a good style builder like the one included in most versions of Visual Studio including the free Visual Web Developer Express Edition. I also recommend the free Internet Explorer Developer Toolbar to help find those hard to locate style elements. Skinner includes an inspector tool that helps with this, but the toolbar is a great help when the built in inspector comes up short!