CSS – cascading style sheets – is a programming language you use to define how things should look like.
It’s pretty basic in all web-development, but still very powerfull. If you have to choose one language to learn this would be a good start – maybe after HTML.
With CSS you can define how all the headlines on your website should look like, how the text should look in all your articles and if the links on your website should be black, blue, white or gray (or any other color).
I’m building BeyondBrussels.com in WordPress, and here it is extremely helpful to be able to do some CSS. I can a little bit, and now the basics.
Basically you can change all the look of your WordPress by knowing some CSS. In every theme made for WordPress there is a CSS-page, where you can change the standard appearance of your website. This CSS-file is included on every other pages. This means that you don’t have to make changes on every little file on your website if you – for example want a bigger headline on your stories.
To ensure some standards in this coding language their is a community called W3. They have made some standards in CSS, to be sure that if you follow them, your website will be showed exactly as want it regardless what browser your visitors are using.
You can read a bit more about W3 on this website: http://www.w3.org/