Css-div is basically a code that the people on this forum use for their signatures. It can be used for special effects on your banners or to put a background that's a different colour than the default and much, much more. There are loads and loads of people using css-div on this forum, so I would advise you to ask some people for the codes that they are using.
If you are going to end up using it, knowing the HTML/Hex codes for colours comes in handy.