A preprocessor is a program that processes its input data to produce output that is used as input to another program. – wikipedia
In web design we use a lot of CSS preprocessors like Sass, LESS, or stylus. A preprocessor converts these raw input data and outputs regular CSS that a web browser can read and use.
But why use a preprocessor like Sass to begin with? Well you can think of it like a Sega CD or expansion pack shown below. Those addon devices extend the capabilities of a Sega Genesis to make it more awesome and fun to use. That is essentially what Sass does for developers. It lets us write CSS in a faster more reusable way because we know it will eventually become regular CSS.