This is simply https://github.com/Automattic/_s style.css broken up into .scss files. Its purpose is to add another 20 minutes to underscores 1000 hour head start.
There are two files added that are blank that I use in every project:
- _mixins.scss
- _mediaqueries.scss
Feel free to add your sass mixins and your media queries accordingly!