PLEASE NOTE: Omega 4.x does not require that you use Sass or any of its related frameworks, however the ease of creating layouts and the time saved while doing so by using Sass is invaluable. Therefore it is not only encouraged that you embrace Sass, but also these other development tools for managing your local environment and Ruby gems.
What's Sass, Ruby, and Ruby gems and why do I need them?
Sass is an extension of CSS3, adding nested rules, variables, mixins, selector inheritance, and more. It’s translated to well-formatted, standard CSS using the command line tool or a web-framework plugin and it is compiled with Ruby. Ruby is the programming language behind Sass and all the things that extend it. Ruby Gems like Compass, Susy, and Singularity GS, are basically software packages written in Ruby.
Sass and all the other Ruby Gems that extend its functionality are development tools that you install on your (optimally local) development machine. You then compile your stylesheets from .scss/.sass to .css from the command line.
Development setup and the software requirements.