Sass stands for Syntactically Awesome Style Sheets. It is a CSS preprocessor. The syntax is .scss (example style.scss) and output file can be style.css.
How to compile?
There are many tools for compiling the Sass which are paid and free.

How to use it?

Using is a pretty easy way. See the Sass Documentation(http://sass-lang.com/documentation/file.SASS_REFERENCE.html) for additional references and examples.