4/10, same old same old.
CSS(Cascading Style Sheets) is originally an html coding style to create sheets with background colors/images and for-ground text and, er, stuff. The original code is <div style>, and vBulletin allows for BBCode usage(Actually, you have to set the HTML replacement yourself, so it's a custom BBCode, but anyways)...
It allows to place stuff over other stuff and to adjust placement and virtual leveling. There are some pretty good threads around the forums that can help you understand it a bit better :)