It's SCSS, a language that compiles to CSS. That whole section has almost nothing to do with CSS, though. That's array syntax for holding a set of utilities to be used later in the code.

It's Sass. Compiles to CSS.

It !defaults a $utilities variable to an empty map (if it's not already set to something) and then merges some things into it.

If I had to guess, the snippet is showing you how to declaratively control how width and margin will change for a give class as it's resized.