Sass (Syntactically Awesome StyleSheets) is an extension of CSS. Sass allows you to use things like variables, nested rules, inline imports and more. It also helps to keep things organized and allows you to create stylesheets faster.
The only requirement for using Sass is that you must have Ruby installed. They also ask that you follow the Sass Community Guidelines.
SASS is an example of a scripting language that extends CSS by allowing developers to write code in one language and then compile it into CSS.
Source: Sass: Install Sass