SASS makes CSS way easier. You don’t have to type the same things over and over. You don’t have to hunt down instances of colors to change the theme of your website. You don’t have to hunt down font definitions to change them. You get variables and functions right in your CSS to make everything easier, faster and cleaner. In the past CSS was actually way more time consuming than writing the backend code that handled the websites/apps requests. Times have changed.
Sass Basics – basic intro and documentation
Sass vs. SCSS: which syntax is better? – an article that discusses the history of SASS and the .scss vs .sass file extensions.
Learn Sass In 20 Minutes | Sass Crash Course