Sass is a CSS extension language that adds functionalities to CSS such as nested inheritance, variables, mixins, functions and plenty more. Sass is actively developed and improving at a fast pace, often adding new features far ahead of other preprocessors.
Sass makes CSS more readable, reusable, and faster to edit. It’s currently the most popular preprocessor out there, with a large community of users and contributors.
There are plenty of frameworks available that will add functionality to Sass. Compass and Bourbon for example, which will take care of vendor prefixes.
There are plenty of resources available for learning Sass, in a wide range of mediums and depth. Which one to pick is dependent on how the learner best absorbs new material and the depth in which they want to learn.
For more information on the benefits of Sass and other preprocessors, check out the “What are the best css preprocessors” question.