Basically, SCSS is the most popular CSS pre-processor now people use. It's just a more recent version of the original Sass syntax. All Sass/SCSS code compiles back to standard CSS so that the browsers can understand it. All browsers currently don't have direct support for SCSS or any other CSS pre-processor, nor does the standard CSS specification provide alternatives for similar features, not yet.
In the web environment, the user can have their browser window set to any number of sizes, and the CSS has to apply to it. Also, users can resize the page after it's opened, and the CSS needs to adjust to new constrains. This means that the browser must calculate those when the page is rendered onscreen. Relative units are one of the tools CSS provides to work for the responsive design.