This action (which is officially named the "IBM Cascading Style Sheets Action" *whew*) is a little hack to make it easier to test stylesheets in MoinMoin by returning the text of a wiki page with MIMEtype 'text/css' so that a standards-compliant browser will treat it as a stylesheet.
Use it by creating the stylesheet under test as a normal wiki page (let's call it TestStyle). If your wiki's address is wiki.example.com, you would specify:
in your Preferences as the "User CSS" URL.
You can then edit the stylesheet in the wiki and test it out easily -- this is especially useful if you don't want to give the person creating the stylesheet access to the wiki server itself.