What the hell is a "font-variation-settings"?

In my bootstrap variables.scss file, I've got this line by default:

$headings-font-weight:        400 !default;

Changing that to 600 or 700 makes the headings on the site bold. Changing this number is how I've controlled the font size for years and years. 

Today, I plugged this new font Mae into a site.  The directions say to use 

font-variation-settings: "wght" 600;

to control the weight.

I tried this on the stylesheet:

$headings-font-weight:        "wght" 600 !default;

Predictably, it did not work. I do not know how to code this font into my variables.scss file.