SASS adds extra functionality to CSS, such as variables, partials (similar to Includes) and other nice stuff. You should really check out this tutorial for more. We use it with a Ruby on Rails app, so I don't have to worry about making it work (although my understanding is that it's quite easy to get going), I just get to use it. Variables are my favourite part, especially for setting colours for use throughout the site. Partials are also very useful too.
Having someone explain the use of something is a lot different to reading a tutorial. I learned quite a few things about SASS and picked up some nice tips which I'll probably start use tomorrow.