It is a program that lets you generate css from the preprocessors own unique syntax.
some of theadvantages are that it has the Ability to add variables, mixins, functions, etc, also Join Multiple Files
Some of the disadvantage are that Aswell as can be really slow even when using the best tools on the fastest computer. Also can produce very large css files
List of at least FIVE CSS Preprocessors currently available.
3. CSS crush
Research Web Development jobs and see how many of them list a CSS Preprocessor as one of their desired skills