A preprocessor is a program that takes one type of data and converts it to another type of data.
In the case of HTML and CSS, some of the more popular preprocessor languages include Haml and Sass.
Haml is processed into HTML and Sass is processed into CSS.
Some advantages to using preprocessors include avoiding repititions, joining multiple files, nested syntax, and ability to add variables, mixins, functions, etc.
Preprocesors also come with some disadvantages, such as decoding being challenging, maintanence and overengineering, and little general understanding of the systems.
Common Proprocessors include:
Quite a few of the jobs I looked at desire skill with preprocessors from employees, and these were just the ones that listed it within the first paragraph.
Being able to use a preprocessor is a very useful skill for getting hired.