CSS is used by web designers to style various elements of websites, from fonts to page layouts.

It is good for reducing the code to content ratio on a site, which is always good for optimisation purposes.

However, there are some unethical designers and optimisers who will use CSS for the purpose of hiding content on their site from viewers, stuffing the pages of a site full of keywords, and cloaking areas of the page.

Search engines do not want to be tricked by web designers into putting unworthy pages higher up its results for particular queries. So the search engines and Google in particular have started indexing a sites CSS files, and penalise a site accordingly if they find spammy techniques in use in this file.

There are legitimate uses for some techniques search engines consider spammy, and it is hard to judge sometimes whether or not a technique is spamming the search engines or not. If you can justify what you do is for your users, you should be ok.

Just dont try and cheat the search engines in this way and you should be ok.