CSS pre-processors process code written in a pre-processed language, such as LESS or Sass, and convert it into standard CSS, which in turn can be interpreted by any standard web browser. CSS pre-processors extend CSS by adding features that allow variables, mixins, and functions.

The benefits of using CSS pre-processors are that they are not bound by any limitations of CSS. CSS pre-processors can give you more functionality and control over your style sheets and allow you to write more maintainable, flexible, and extendable CSS.

CSS pre-processors can also help to reduce the amount of CSS and assist with the management of large and complex style sheets that can become harder to maintain as the size and complexity increases.

In essence, CSS pre-processors such as Less and Sass enable programmatic control over your style sheets.

Bootstrap moved their source files from Less to Sass with version 4. Less and Sass are very alike in that they share a similar syntax as well as features such as variables, mixins, partials, and nesting, to name but a few.

Less was influenced by Sass, and later on, Sass was influenced by Less when it adopted CSS-like block formatting, which worked very well for Less.