Learning the basics of Sass
Sass stands for Syntactically Awesome Style Sheets. If you've never used or heard of Sass before, it's a CSS preprocessor. A preprocessor extends regular CSS by allowing the use of things such as variables, operators, and mixins in CSS. Sass is written during the development stage of your project and it needs to be compiled into regular CSS before you deploy your project into production. I'll cover that in more detail in the next section but don't worry because Harp.js makes this really easy to do.
Up until version 4 of Bootstrap, the CSS preprocessor used was actually Less. For a good while both Sass and Less were popular in frontend design circles. However, over the last few years, while Sass has emerged as the best choice for developers, the Bootstrap team decided to make the change in version 4. If you are familiar with Less but have never used Sass, don't worry as they are pretty similar to use so it won't take much to get you up-to-speed.