SASS
In plain terms
A CSS preprocessor adding variables, nesting, and reuse.
Going deeper
Sass extends CSS with programmable features like variables, mixins, and nesting, compiling down to plain CSS. It makes large stylesheets more maintainable.
Under the hood
Mixins and functions reduce repetition; the SCSS syntax is a CSS superset, easing adoption.
Examples
Theming, design systems, large stylesheets, Bootstrap's source.
Think of it like...
Like giving plain CSS a toolkit of shortcuts and reusable parts before it ships.