Sass is a CSS extension language. You can write your CSS with the extra tools and functionality that Sass gives you, and your code is tranlated into plain CSS for the browser.
Reasons to love Sass:
It is easy to learn and start using right away.
You can break your CSS up into multiple files but still only have one HTTP request. More files allows for more organized and maintainable code.
You can use variables. It is easy to change a color or a value in multiple places.
You have access to helper functions provided by Sass and you can write your own functions to do exactly what you want.