CSS preprocessing using SASS

NOTE: This is an excerpt from https://www.drupal.org/docs/7/theming/working-with-css/sass-techniques-a... page. Please refer to it for the complete documentation.

Sass is a meta-language on top of CSS that’s used to describe the style of a document cleanly and structurally, with more power than flat CSS allows.

Sass provides a simpler, more elegant syntax for CSS and implements various features that are useful for creating manageable stylesheets -- such as variables, functions, nested rules and more. See the Sass documentation for details.

Here is an article on Sass CSS preprocessor that talks about everything you need to get started with Sass.