What does a CSS Preprocessor do?

It is a program that lets you generate css from the preprocessors own unique syntax.

What are the advantages of using a CSS Preprocessor?

some of theadvantages are that it has the Ability to add variables, mixins, functions, etc, also Join Multiple Files

What are the disadvantages or issues you need to consider when using a CSS Preprocessor?

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.
2. Less
3. CSS crush
4. Stylus
5. Compass

Research Web Development jobs and see how many of them list a CSS Preprocessor as one of their desired skills

So far I cannot find at all any job that has css preprocesser as are required but I am pretty sure you are supposed to have that cause its used a lot in web development.