What is and how to use Sass and Npm in Bootstrap 4?

Sass (Syntactically Awesome StyleSheets) - Is an extension of CSS. A CSS preprocessor. In fact it's basically a "language" that is transliterated into CSS. You can read in detail here: Sass - reference

The advantage of using SASS is that it adds programming elements that do not exist in CSS, such as using variables and Procedures (called MIXINS). With this, SASS organizes CSS in a procedural way.

NPM (Node Package Manager) - This is a package distribution utility. Similiar to NuGet (.NET) and Mavem (Java). Here's a better description here: What is NPM and Node?

NPM today is the most used utility (today in 2018) for distribution and maintenance of packages under WEB development.

Now how does this relate to Bootstrap4?

Bootstrap is a CSS framework. In previous versions LESS (equivalent to SASS) was used as a CSS preprocessor. SASS is less complex than LESS (see a reference here: SASS vs LESS ) was then chosen as default .

One way to integrate Bootstrap4 into your project is to use NPM. This greatly simplifies the development process.

If you want to know more about Bootstrap 4 news, I recommend this article: Bootstrap: The v4 changes